Disclaimer: This is a rough guide, I don't consider myself an expert at D&D, I did this just for fun and to pass the time. There are likely many mistakes in here, both with calculations and spelling/grammar, so I apologise for that.

Welcome to my guide to pets as sidekicks, this is basically a big list telling you how good in combat the playful wolf, happy horse or undead horror you or your party have adopted would be if you trained them to be a sidekick. Tasha's limitations unfortunately means no training your adopted Wyrmling or Giant Scorpion as a sidekick, but some of the pets you train can easily become stronger than this. This guide includes most pet's you'd want to have fighting alongside you and a few you might not have considered. I have estimated each pets HP,  Natural AC, Damage and Challenge Rating at levels 5, 10 and 20 for each pet. I only have statblocks from the Monster Manual, Volo's, Mordenkainen's and a few other's from other modules. Challenge Rating was calculated using the table in the DMG and as such should only be treated as a rough indicator of how good one of these sidekicks is in combat.

Rules

For this guide, I defined a pet that can be a sidekick as the following: Any creature who has a CR less than 1 and higher than 0, possesses natural weapons and does not possess the ability to hold weapons (I'm sure people will argue over whether these creatures can hold weapons, but I used my own judgement). This includes some rather fanciful options that many DM's will most likely not allow, but they are there anyway for fun. I didn't do creatures with a CR of 0, because that would've doubled the amount of work I would have had to do and I feel like most parties want pet sidekicks that can actually do decent damage.

To calculate what the pet's abilities would be like at higher levels, all pets (unless otherwise specified) have had their attack modifier prioritised first when levelling up, followed by Constitution, followed by Dex(If Str was the attack modifier). I have chosen  the option +2 to attack rolls and in cases when it's not clear whether an attack uses dex or str as their attack modifier, I have assumed dex. If there is a range in the CR, the highest number in the range is it's CR with the best possible non-magical armour as barding, and the lowest number is its CR using natural ac alone, if there isn't a range in the CR it either means armour makes little difference to the CR or it can't reasonably wear armour. I have assumed the following when calculating this; That flying creatures can only wear light armour so they can still fly (Like the rules for Aarakocra), that sea creatures don't wear barding as it would effect their ability to move in the water and that certain creatures can't wear armour (example: oozes).

Without further ado, let's begin

Template Monster Name - Movement -  Sourcebook Found in - Rating (0-5)
Level 5 - Average Max HP - Natural AC - Hit Chance - Average Damage/Round - Estimated Challenge Rating
Level 10 - Average Max HP - Natural AC - Hit Chance - Average Damage/Round - Estimated Challenge Rating
Level 20 - Average Max HP - Natural AC - Hit Chance - Average Damage/Round - Estimated Challenge Rating
Comments: ""

Abyssal Wretch - 20ft. - Mordenkainen's Tome of Foes - 1/5
Level 5 - 40hp - 12ac - +7 to hit - 6 damage - 1CR
Level 10 - 63hp - 14ac - +9 to hit - 15 damage - 3CR
Level 20 - 148hp - 16ac - +13 to hit - 28 damage - 8-9CR
Comments: This is a terrible choice; it's slow, it's got low hp and, as it's stupid and designed to attack non-demons on sight, will probably not be allowed by any sane DM

Ambush Drake - 30ft. - Hoard of the Dragon Queen - 2/5
Level 5 - 49hp - 14ac - +8 to hit - 7 damage - 1-2CR
Level 10 - 77hp - 16ac - +10 to hit - 12 damage - 4-5CR
Level 20 - 176hp - 17ac - +13 to hit - 18 damage - 9-10CR
Comments: This creature is annoying as its attack modifier is dex, but its damage modifier is str (At least in my physical copy of the stat block). It has pack tactics and ambush which makes it nice at low levels, but you'll have to choose between hp or damage at higher levels.

Ape - 30ft. Climb: 30ft. - Basic Rules - 4/5
Level 5 - 49hp - 12ac - +9 to hit - 15 damage - 2-3CR
Level 10 - 84hp - 13ac - +11 to hit - 17 damage - 4-5CR
Level 20 - 209hp - 14ac - +13 to hit - 25 damage - 9-10CR
Comments: Did you know Ape's have better stats than black bears? This is a very good sidekick at low levels and is also one of the few with a ranged attack making it very useful.

Axe Beak - 50ft. - Basic Rules - 3/5
Level 5 - 52hp - 11ac - +8 to hit - 7 damage - 1-2CR
Level 10 - 84hp - 12ac - +10 to hit - 17 damage - 3-5CR
Level 20 - 209hp - 12ac - +13 to hit - 28 damage - 9-10CR
Comments: If you like Chocobo's, you'll love this guy. Does alright damage for a mount but pales in comparison to the Horses

Black Bear - 40ft. Climb: 30ft. - Basic Rules - 3/5
Level 5 - 36hp - 11ac - +8 to hit - 14 damage - 1-2CR
Level 10 - 84hp - 12ac - +10 to hit - 18 damage - 4-5CR
Level 20 - 209hp - 12ac - +13 to hit - 30 damage - 9-10CR
Comments: It looks alright, until you compare it with the Ape, which has better stats and a ranged attack. Strong at low levels due to multi-attack

Blink Dog - 40ft. - Basic Rules - 3/5
Level 5 - 49hp - 13ac - +7 to hit - 5 damage - 1/2-1CR
Level 10 - 77hp - 14ac - +9 to hit - 13 damage - 3-4CR
Level 20 - 172hp - 14ac - +13 to hit - 25 damage - 9CR
Comments: Ok damage and hp, however it can also teleport which is pretty useful. Good companion for an Archfey Warlock or Fey Wanderer

Blood Hawk - 10ft. Fly: 60ft. - Basic Rules - 2/5
Level 5 - 24hp - 13ac - +8 to hit - 5 damage - 1CR
Level 10 - 42hp - 15ac - +10 to hit - 13 damage - 3CR
Level 20 - 137hp - 16ac - +13 to hit - 22 damage - 8CR
Comments: It's an okay flying sidekick, but it has very low hp.

Boggle - 30ft. Climb: 30ft. - Volo's Guide to Monsters - 2/5
Level 5 - 40hp - 15ac - +2 to hit - 2 damage - 1/2-1CR
Level 10 - 66hp - 16ac - +3 to hit - 5 damage - 1CR
Level 20 - 188hp - 16ac - +5 to hit - 7 damage - 8CR
Comments: I used a different ASI increase for the boggle, focusing on increasing it's Nat AC and Constitution because this is a much better support sidekick than a damage dealer. It can make invisible portals only it can see and go through, and can also help lay ambushes by filling hallways with oil puddles.

Boar - 40ft. - Basic Rules - 3/5
Level 5 - 38hp - 12ac - +7 to hit - 6 damage - 1CR
Level 10 - 66hp - 13ac - +9 to hit - 14 damage - 3-4CR
Level 20 - 161hp - 13ac - +13 to hit - 26 damage - 8-9CR
Comments: Not great, it can knock enemies prone, but so can wolves which are much better.

Camel - 50ft. - Basic Rules - 1/5
Level 5 - 52hp - 9ac - +9 to hit - 2 damage - 1/2CR
Level 10 - 90hp - 10ac - +11 to hit - 5 damage - 2-4CR
Level 20 - 214hp - 11ac - +13 to hit - 7 damage - 7-9CR
Comments: It's pretty poor, its only attack doesn't include its modifier in the damage and it starts with 9ac. Even the mule is better than this.

Cockatrice - 20ft. Fly: 40ft. - Basic Rules - 4/5
Level 5 - 49hp - 12ac - +7 to hit - 4 damage - 1/2-1CR
Level 10 - 72hp - 14ac - +9 to hit - 11 damage - 3-4CR
Level 20 - 157hp - 16ac - +13 to hit - 22 damage - 9CR
Comments: Each attack results in a con save, making an enemy restrained on failure, then petrified on double failure. This makes it quite powerful against enemies with low con.

Constrictor Snake - 30ft. Swim: 30ft. - Basic Rules - 4/5
Level 5 - 45hp - 12ac - +8 to hit - 7 damage - 1-2CR
Level 10 - 78hp - 14ac - +10 to hit - 17 damage - 4-5CR
Level 20 - 203hp - 16ac - +13 to hit - 28 damage - 10CR
Comments: Its attack automatically restrains a target on a hit, taking it out of the fight. Alright damage, good nat ac and good hp as well

Cow/Ox/Rothe - 30ft. - Volo's Guide to Monsters - 4/5
Level 5 - 52hp - 10ac - +10 to hit - 10 damage - 2-3CR
Level 10 - 100hp - 11ac - +11 to hit - 19 damage - 4-6CR
Level 20 - 225hp - 13ac - +13 to hit - 27 damage - 10-11CR
Comments: All 3 use the same stat block with minor differences. Give these cattle some armour and you have a decently strong tank, especially in the early levels

Crocodile - 20ft - Basic Rules - 3/5
Level 5 - 57hp - 12ac - +8 to hit - 8 damage - 1-2CR
Level 10 - 94hp - 13ac - +10 to hit - 19 damage - 3-4CR
Level 20 - 229hp - 13ac - +13 to hit - 31 damage - 10-11CR
Comments: Strong natural ac, decent hp and automatically restrains on a hit, however the speed is a let down. The dimetrodon is a much better crocodile like creature.

Darkmantle - 10ft. Fly: 30ft. - Basic Rules - 2 or 3/5
Level 5 - 45hp - 11ac - +9 to hit - 7 damage - 1CR
Level 10 - 67hp - 12ac - +11 to hit - 17 damage - 3CR
Level 20 - 192hp - 12ac - +13 to hit - 25 damage - 8CR
Comments: This becomes 3/5 if your DM uses the flanking rule, as this allows it to get advantage in combat more easily, which is required for it to blind other creatures.

Dimetrodon - 30ft. Swim: 20ft. - Volo's Guide to Monsters - 4/5
Level 5 - 52hp - 12ac - +8 to hit - 10 damage - 1-2CR
Level 10 - 84hp - 13ac - +10 to hit - 22 damage - 4-5CR
Level 20 - 209hp - 13ac - +13 to hit - 36 damage - 9-10CR
Comments: This is basically a higher damage dealing, faster crocodile. It does good damage at all levels, doesn't get in the way during combat and has decent hp.

Dolphin - 0ft. Swim: 50ft. - Volo's Guide to Monsters - 1/5
Level 5 - 38hp - 11ac - +8 to hit - 7 damage - 1CR
Level 10 - 66hp - 12ac - +10 to hit - 16 damage - 3CR
Level 20 - 181hp - 13ac - +13 to hit - 26 damage - 8CR
Comments: Can't walk on land and has to resurface for air every 20 minutes. If you're doing an underwater campaign, use a Reef Shark instead.

Draft Horse - 40ft. - Basic Rules - 4/5
Level 5 - 52hp - 10ac - +10 to hit - 10 damage - 2-3CR
Level 10 - 94hp - 11ac - +11 to hit - 20 damage - 4-5CR
Level 20 - 229hp - 12ac - +13 to hit - 36 damage - 9-11CR
Comments: Tankier, slower riding horse. All the horses have decent hp and do decent damage.

Dretch - 20ft. - Basic Rules - 2/5
Level 5 - 40hp - 12ac - +6 to hit - 10 damage - 2CR
Level 10 - 63hp - 14ac - +8 to hit - 14 damage - 4-5CR
Level 20 - 128hp - 17ac - +13 to hit - 30 damage - 9CR
Comments: The best fiend sidekick (which isn't saying much), it does decent damage at low levels, but its low attack modifier and hp hinder it. Convince a demon lord to promote it to a Balor however and you suddenly have a very powerful sidekick, though good luck convincing your DM.

Elk - 50ft. - Basic Rules - 4/5
Level 5 - 45hp - 10ac - +9 to hit - 12 damage - 2-3CR
Level 10 - 78hp - 11ac - +11 to hit - 23 damage - 3-5CR
Level 20 - 223hp - 11ac - +13 to hit - 33 damage - 9-11CR
Comments: A Riding Horse that exchanges 10ft of movement for a charge attack. Does very good damage at low levels and stays decent at higher levels. If you ever do a Xmas campaign, you can use the statblock for a reindeer sidekick.

Flumph - 5ft. Fly: 30ft. - Monster Manual - 3/5
Level 5 - 24hp - 13ac - +8 to hit - 10 damage - 1CR
Level 10 - 42hp - 15ac - +10 to hit - 20 damage - 3CR
Level 20 - 137hp - 16ac - +13 to hit - 32 damage - 8CR
Comments: If an enemy frequently makes its con saves then the Flump's damage is about 30-40% lower. Has many good social abilities such as being able to intercept telepathy. However it has low hp and requires psionic energy to survive, which means it'll starve to death if you don't have a psionics subclass in your party.

Flying Snake - 30ft. Fly: 60ft. Swim: 30ft. - Basic Rules - 3/5
Level 5 - 17hp - 15ac - +10 to hit - 8 damage - 1CR
Level 10 - 40hp - 16ac - +11 to hit - 17 damage - 3-4CR
Level 20 - 155hp - 16ac - +13 to hit - 25 damage - 8CR
Comments: Cute, does a surprising amount of damage against enemies without poison resistance or immunity and can fly. However its low hp is a problem

Flying Sword - 0ft. Fly: 50ft. - Basic Rules - 3/5
Level 5 - 35hp - 17ac - +7 to hit - 6 damage - 2CR
Level 10 - 52hp - 18ac - +9 to hit - 15 damage - 4CR
Level 20 - 127hp - 19ac - +13 to hit - 28 damage - 8CR
Comments: It can fly, has very good natural ac and you don't need to feed it. However it has very low hp.

Giant Badger - 30ft. Burrow: 10ft. - Basic Rules - 3/5
Level 5 - 50hp - 10ac - +7 to hit - 12 damage - 1-2CR
Level 10 - 88hp - 11ac - +9 to hit - 16 damage - 3-5CR
Level 20 - 203hp - 11ac - +13 to hit - 30 damage - 9-10CR
Comments: Decent damage but a low attack modifier and natural ac. Is made better when given barding. Its ability to burrow could be useful for a party looking for alternative entrances.

Giant Bat - 10ft. Fly: 60ft. - Basic Rules - 3/5
Level 5 - 54hp - 13ac - +8 to hit - 6 damage - 1-2CR
Level 10 - 77hp - 14ac - +10 to hit - 15 damage - 3-4CR
Level 20 - 212hp - 14ac - +13 to hit - 25 damage - 9-10CR
Comments: Its strength comes in also being a flying mount, and having decent hp for a flier. If you're looking for a flying mount that does more damage, check out the Giant Owl.

Giant Centipede - 30ft. Climb: 30ft. - Basic Rules - 4/5
Level 5 - 27hp - 14ac - +8 to hit - 15 damage - 2CR
Level 10 - 49hp - 16ac - +10 to hit - 34 damage - 4CR
Level 20 - 154hp - 17ac - +13 to hit - 53 damage - 10CR
Comments: Does huge damage, although 30-60% of this can be negated by con saves or poison immunity. Strong natural ac but low hp until you start putting points into con. If you think this is good, wait until you see the giant wasp.

Giant Crab - 30ft. Climb: 30ft. - Basic Rules - 3/5
Level 5 - 36hp - 16ac - +7 to hit - 5 damage - 1/2CR
Level 10 - 58hp - 17ac - +9 to hit - 13 damage - 3-4CR
Level 20 - 143hp - 17ac - +13 to hit - 25 damage - 8CR
Comments: Once it gains extra attack, it becomes able to grapple 2 enemies each turn, also has decent natural ac. Let down by its low hp.

Giant Frog - 30ft. Swim: 30ft. - Basic Rules - 3/5
Level 5 - 45hp - 12ac - +7 to hit - 8 damage - 1-2CR
Level 10 - 73hp - 14ac - +9 to hit - 16 damage - 3-5CR
Level 20 - 168hp - 16ac - +13 to hit - 28 damage - 9CR
Comments: Alright damage, good natural ac, can swallow small creatures which makes them good against goblins/kobolds/children

Giant Goat - 40ft. - Basic Rules - 4/5
Level 5 - 41hp - 12ac - +9 to hit - 10 damage - 1-2CR
Level 10 - 73hp - 13ac - +11 to hit - 21 damage - 4-5CR
Level 20 - 218hp - 13ac - +13 to hit - 31 damage - 10-11CR
Comments: Pretty balanced defensively and offensively, has a charge attack that can knock enemies prone and if you get a female, provides free milk.

Giant Lizard - 30ft. Climb: 30ft. - Basic Rules - 3/5
Level 5 - 57hp - 12ac - +8 to hit - 7 damage - 1-2CR
Level 10 - 94hp - 13ac - +10 to hit - 17 damage - 4-5CR
Level 20 - 229hp - 13ac - +13 to hit - 28 damage - 10-11CR
Comments: A weaker draft horse that has scales and can climb up walls.

Giant Owl - 5ft. Fly: 60ft. - Basic Rules - 4/5
Level 5 - 52hp - 13ac - +7 to hit - 9 damage - 2CR
Level 10 - 84hp - 14ac - +9 to hit - 20 damage - 4-5CR
Level 20 - 189hp - 14ac - +13 to hit - 36 damage - 9-10CR
Comments: The best flying sidekick that could be used as a mount, the flight makes it very strong in the early levels and continue to deal decent damage as it levels.

Giant Poisonous Snake - 30ft. Swim: 30ft. - Basic Rules - 4/5
Level 5 - 38hp - 15ac - +10 to hit - 17 damage - 2-3CR
Level 10 - 76hp - 16ac - +11 to hit - 35 damage - 5CR
Level 20 - 201hp - 16ac - +13 to hit - 53 damage - 11-12CR
Comments: It should be noted 30% of its damage comes from requiring enemies to fail con saves. Very good modifier and natural armour however.

Giant Rat - 30ft. - Basic Rules - 2/5
Level 5 - 29hp - 13ac - +8 to hit - 5 damage - 1CR
Level 10 - 52hp - 15ac - +10 to hit - 13 damage - 2-3CR
Level 20 - 157hp - 16ac - +13 to hit - 22 damage - 8CR
Comments: low hp, low damage. Could be used for scouting.

Giant Wasp - 10ft. Fly: 50ft. - Basic Rules - 5/5
Level 5 - 36hp - 13ac - +8 to hit - 16 damage - 2CR
Level 10 - 58hp - 15ac - +10 to hit - 36 damage - 3-4CR
Level 20 - 163hp - 16ac - +13 to hit - 56 damage - 10CR
Comments: This is a flying version of the Giant Poisonous Snake, incredibly strong in low-mid levels and is still a worthy fighter at high levels. Only downside is poison immunity becomes common at higher levels, wiping out 60% of its damage.

Giant Weasel - 40ft. - Basic Rules - 2/5
Level 5 - 31hp - 14ac - +9 to hit - 5 damage - 1CR
Level 10 - 54hp - 16ac - +11 to hit - 13 damage - 3CR
Level 20 - 179hp - 16ac - +13 to hit - 22 damage - 8-9CR
Comments: A slightly better Giant Rat, small creatures can use this as a mount.

Giant Wolf Spider - 40ft. Climb: 40ft. - Basic Rules - 4/5
Level 5 - 31hp - 13ac - +7 to hit - 12 damage - 1-2CR
Level 10 - 66hp - 14ac - +9 to hit - 27 damage - 3-4CR
Level 20 -170hp - 14ac - +13 to hit - 46 damage - 9-10CR
Comments: Reduce the offensive ability of the Giant Poisonous Snake and give it higher speed and a climbing speed and you get the spider. Still does very decent damage and could work well as a companion for a drow.

Hadrosaurus - 40ft. - Volo's Guide to Monsters - 4/5
Level 5 - 57hp - 11ac - +8 to hit - 8 damage - 1CR
Level 10 - 94hp - 12ac - +10 to hit - 19 damage - 4-5CR
Level 20 - 229hp - 12ac - +13 to hit - 31 damage - 9-11CR
Comments: The reptilian equivalent of the riding horse, has a lot of hp and does pretty good damage.

Juvenile Mimic - 10ft. Climb: 10ft. - Tasha's Cauldron of Everything - 1/5
Level 5 - 24hp - 12ac - +7 to hit - 3 damage - 1/4CR
Level 10 - 42hp - 14ac - +9 to hit - 7 damage - 1CR
Level 20 - 117hp - 16ac - +13 to hit - 10 damage - 6CR
Comments: Did this one as a joke, while a chameleonic sidekick would be useful, the juvenile mimic has terrible speed, hp and damage. A group of lvl 1's could easily take out a lvl 20 one of these.

Kruthik - 30ft. Burrow: 10ft. Climb: 30ft. - Mordenkainen's Tome of Foes - 3/5
Level 5 - 31hp - 17ac - +9 to hit - 6 damage - 2CR
Level 10 - 54hp - 19ac - +11 to hit - 15 damage - 4CR
Level 20 - 179hp - 19ac - +13 to hit - 22 damage - 9CR
Comments: Not a bad little critter, very good nat ac, has tremorsense and can burrow tunnels party members could crawl through.

Magmin - 30ft. - Basic Rules - 3/5
Level 5 - 31hp - 15ac - +8 to hit - 10 damage - 3CR
Level 10 - 54hp - 17ac - +10 to hit - 17 damage - 5CR
Level 20 - 166hp - 18ac - +13 to hit - 24 damage - 10CR
Comments: It has nonmagical melee resistance, and is okay in combat, however it can only deal fire damage. Its CR is also skewed by the fact it explodes on death, something a sidekick with death saving throws is unlikely to be doing a lot.

Manes - 20ft. - Monster Manual - 1/5
Level 5 - 31hp - 9ac - +6 to hit - 6 damage - 1/2CR
Level 10 - 54hp - 10ac - +8 to hit - 14 damage - 1-2CR
Level 20 - 126hp - 10ac - +13 to hit - 27 damage - 6-8CR
Comments: One of the worst picks in this list, if you really want a demon sidekick you're better off going with the dretch.

Mastiff - 40ft. - Basic Rules - 3/5
Level 5 - 33hp - 12ac - +7 to hit - 5 damage - 1/2-1CR
Level 10 - 60hp - 13ac - +9 to hit - 13 damage - 2-4CR
Level 20 - 155hp - 13ac - +13 to hit - 25 damage - 7-8CR
Comments: This is the dog sidekick, it has pretty low hp and low damage but can knock enemies prone and has decent speed. If you want a better good boi sidekick, then use the wolf statblock instead.

Mule - 40ft. - Basic Rules - 3/5
Level 5 - 38hp - 10ac - +8 to hit - 5 damage - 1/2-1CR
Level 10 - 66hp - 11ac - +10 to hit - 13 damage - 2-4CR
Level 20 - 181hp - 11ac - +13 to hit - 22 damage - 7-9CR
Comments: Worst of the equine sidekicks, but has alright hp and does ok damage.

Needle Blight - 30ft. - Monster Manual - 5/5
Level 5 - 38hp - 13ac - +7 to hit - 9 damage - 1CR
Level 10 - 66hp - 15ac - +9 to hit - 20 damage - 3CR
Level 20 - 161hp - 17ac - +13 to hit - 36 damage - 9CR
Comments: You might be wondering why this one scored a 5/5, because the Needle Blight's attacks are ranged. This is the best ranged attacker here and it's damage overall is pretty great. Because of the nature of its attack, I don't believe it's capable of wearing armour.

Neogi Hatchling - 20ft. Climb: 20ft. - Volo's Guide to Monsters - 3/5
Level 5 - 20hp - 12ac - +7 to hit - 11 damage - 1CR
Level 10 - 32hp - 14ac - +9 to hit - 25 damage - 2-3CR
Level 20 - 97hp - 16ac - +13 to hit - 43 damage - 8CR
Comments: The damage isn't even reliant on failing con saves, however half of its damage is poison so less useful at higher levels. It is slow and has really low hp so you may want to consider a Giant Wolf Spider instead.

Nupperibo - 20ft. - Mordenkainen's Tome of Foes - 1/5
Level 5 - 38hp - 12ac - +9 to hit - 11 damage - 3CR
Level 10 - 66hp - 14ac - +11 to hit - 21 damage - 5CR
Level 20 - 201hp - 15ac - +13 to hit - 29 damage - 11CR
Comments: You may look at the damage and go, why is that only 1/5. That's because this particular devil can't see anything more than 10ft away and will deal piercing damage against you and your party members every turn if you stand too close.

Oblex Spawn - 20ft. - Mordenkainen's Tome of Foes - 3/5
Level 5 - 40hp - 14ac - +9 to hit - 8 damage - 1CR
Level 10 - 63hp - 16ac - +11 to hit - 20 damage - 4CR
Level 20 - 168hp - 16ac - +13 to hit - 29 damage - 9CR
Comments: Surprisingly decent, alright hp, very good damage, also deals psychic damage. A warlock whose patron is Juiblex might be given one of these and tasked to help it grow. May cause bouts of amnesia amongst your party.

Panther - 50ft Climb: 40ft. - Basic Rules - 3/5
Level 5 - 36hp - 13ac - +8 to hit - 9 damage - 1CR
Level 10 - 58hp - 15ac - +10 to hit - 17 damage - 3-4CR
Level 20 - 163hp - 16ac - +13 to hit - 28 damage - 7-8CR
Comments: Decent speed and damage, HP isn't great however and it has low constitution.

Pony - 40ft. - Basic Rules - 3/5
Level 5 - 43hp - 10ac - +8 to hit - 8 damage - 1-2CR
Level 10 - 76hp - 11ac - +10 to hit - 18 damage - 3-5CR
Level 20 - 201hp - 11ac - +13 to hit - 30 damage - 9-10CR
Comments: Fairly balanced, does decent damage. The horses are better, but you can use this for that My Little Pony campaign you've been working on.

Pseudodragon - 15ft. Fly: 60ft. - Basic Rules - 4/5
Level 5 - 29hp - 14ac - +8 to hit - 5 damage - 1/2CR
Level 10 - 52hp - 16ac - +10 to hit - 13 damage - 3CR
Level 20 - 157hp - 17ac - +13 to hit - 22 damage - 9CR
Comments: It's cute, can fly and has a chance with each attack to poison an enemy or knock them unconscious. Not so good on the hp and damage front however.

Psychic Gray Ooze - 10ft. Climb: 10ft. - Monster Manual - 3/5
Level 5 - 60hp - 9ac - +7 to hit - 12 damage - 2CR
Level 10 - 91hp - 10ac - +9 to hit - 27 damage - 5CR
Level 20 - 212hp - 10ac - +13 to hit - 46 damage - 12CR
Comments: Does loads of damage and also corrodes enemy weapons and armour on touch. Has a recharge ranged attack as well. If it wasn't for the terrible speed and ac it'd probably be 5/5.

Pteranodon - 10ft. Fly: 60ft. - Basic Rules - 3/5
Level 5 - 36hp - 13ac - +7 to hit - 7 damage - 1CR
Level 10 - 58hp - 14ac - +9 to hit - 16 damage - 3CR
Level 20 - 143hp - 14ac - +13 to hit - 30 damage - 8CR
Comments: Decent damage but low hp and modifier, however that's to be expected of flying beasts. A small character would be able to ride it.

Reef Shark - 0ft. Swim: 30ft. - Basic Rules - 2/5
Level 5 - 49hp - 12ac - +8 to hit - 7 damage - 1CR
Level 10 - 77hp - 13ac - +10 to hit - 17 damage - 4CR
Level 20 - 193hp - 14ac - +13 to hit - 28 damage - 9CR
Comments: The best water breathing pet. Unfortunately most campaigns take place on land.

Riding Horse - 60ft. - Basic Rules - 4/5
Level 5 - 45hp - 10ac - +9 to hit - 9 damage - 2-3CR
Level 10 - 78hp - 11ac - +11 to hit - 20 damage - 3-5CR
Level 20 - 223hp - 11ac - +13 to hit - 30 damage - 9-11CR
Comments: Why ride your horse into combat, when you could train it to fight alongside you. If given barding can be very good in combat.

Rust Monster - 40ft. - Basic Rules - 4/5
Level 5 - 54hp - 15ac - +7 to hit - 6 damage - 1CR
Level 10 - 82hp - 17ac - +9 to hit - 15 damage - 4CR
Level 20 - 177hp - 19ac - +13 to hit - 28 damage - 10CR
Comments: High natural ac, rusts enemy weapons when attacked and deals alright damage, these things are pretty good. Monster Manual says these guys can be pets so lorewise it's perfectly fine to have one as a companion.

Shadow - 40ft. - Basic Rules - 5/5 (Possibly broken)
Level 5 - 44hp - 13ac - +8 to hit - 10 damage - 3CR
Level 10 - 65hp - 15ac - +10 to hit - 22 damage - 5CR
Level 20 - 186hp - 16ac - +13 to hit - 36 damage - 9CR
Comments: Disgustingly broken. Boasting 8 resistances, 2 immunities, decent speed, decent natural ac, decent damage, darkvision, bonuses to stealth and, most importantly, its attacks drain an enemies strength by 1d4 on each hit. I didn't say 'drains a humanoids strength', this affects almost any creature. And when an enemy reaches 0 strength, they immediately die. I like the idea of having a character who has this as a sidekick, and outside of combat it just hides in said characters shadow.

Slaad Tadpole - 30ft. - Monster Manual - 2/5
Level 5 - 22hp - 13ac - +8 to hit - 5 damage - 1CR
Level 10 - 35hp - 15ac - +10 to hit - 13 damage - 3CR
Level 20 - 120hp - 16ac - +13 to hit - 22 damage - 8CR
Comments: Really, it's practically impossible to have one of these as a sidekick, as they grow into a blue slaad in 12-24 hours, if you somehow manage to train a tadpole to 20 levels in under 12 hours, it'd become a Blue Slaad with the following stats - 333hp - 19ac - +13 to hit - 36 damage - 15CR. It's unlikely your DM will let you have this however.

Star Spawn Grue - 30ft. - Mordenkainen's Tome of Foes - 1/5
Level 5 - 35hp - 12ac - +7 to hit - 7 damage - 1CR
Level 10 - 52hp - 14ac - +9 to hit - 16 damage - 2-3CR
Level 20 - 127hp - 16ac - +13 to hit - 30 damage - 8CR
Comments: This thing gives disadvantage on weapon attacks to all non-aberrations close to it, making it a hindrance. If your party consists entirely of Illithids, Slaad and Beholders however then it becomes 5/5.

Steeder(Male Only) - 30ft. Climb: 30ft. - Mordenkainen's Tome of Foes - 5/5
Level 5 - 45hp - 12ac - +8 to hit - 11 damage - 1-2CR
Level 10 - 78hp - 13ac - +10 to hit - 26 damage - 4-5CR
Level 20 - 203hp - 13ac - +13 to hit - 41 damage - 10-11CR
Comments: A better version of the Giant Wolf Spider as its poison damage isn't reliant on enemies failing con saves. Also has a neat jump ability and can be a mount for small races.

Stench Kow - 30ft. - Volo's Guide to Monsters - 3/5
Level 5 - 38hp - 10ac - +10 to hit - 10 damage - 2-4CR
Level 10 - 100hp - 11ac - +11 to hit - 19 damage - 6-7CR
Level 20 - 214hp - 13ac - +13 to hit - 27 damage - 10-12CR
Comments: This is what devil farmers milk apparently, basically a cow with a stench ability that poisons people around it, so not great if your party likes clean air.

Twig Blight - 20ft. - Basic Rules - 2/5
Level 5 - 27hp - 14ac - +7 to hit - 4 damage - 1/2CR
Level 10 - 49hp - 16ac - +10 to hit -11 damage - 2CR
Level 20 - 154hp - 18ac - +13 to hit - 22 damage - 8CR
Comments: Low damage, Low Hp, slow and becomes ash very quickly when exposed to fire

Vegepygmy - 30ft. - Volo's Guide to Monsters - 3/5
Level 5 - 31hp - 14ac - +8 to hit - 6 damage - 1/2CR
Level 10 - 54hp - 16ac - +10 to hit - 15 damage - 4CR
Level 20 - 159hp - 17ac - +13 to hit - 25 damage - 9CR
Comments: Low hp, but it does have regeneration. Probably could hold a weapon so you could give it a tiny rapier for extra damage

Velociraptor - 30ft. - Volo's Guide to Monsters - 3/5
Level 5 - 28hp - 14ac - +8 to hit - 11 damage - 1CR
Level 10 - 45hp - 16ac - +10 to hit - 15 damage - 3-4CR
Level 20 - 128hp - 17ac - +13 to hit - 25 damage - 8CR
Comments: A lot smaller than the one's in Jurassic Park, still alright damage and good natural ac. These guys do a lot of damage for a tiny creature though.

Vine Blight - 20ft. - Monster Manual - 4/5
Level 5 - 58hp - 12ac - +8 to hit - 10 damage - 1-3CR
Level 10 - 91hp - 13ac - +10 to hit - 22 damage - 4-6CR
Level 20 - 216hp - 13ac - +13 to hit - 36 damage - 10-11CR
Comments: Decent hp, good damage and restrains enemies when it attacks. Let down by its low speed. Is a lot more useful in a forest/jungle setting.

Warhorse - 60ft. - Basic Rules - 5/5
Level 5 - 52hp - 11ac - +10 to hit - 16 damage - 2-3CR
Level 10 - 94hp - 12 ac - +11 to hit - 28 damage - 5-6CR
Level 20 - 229hp - 13ac - +13 to hit - 40 damage - 10-11CR
Comments: This beast is one of the best sidekicks you can get, strong damage, great hp, great modifier. If given barding becomes even better.

Wolf - 40ft. - Basic Rules - 4/5
Level 5 - 38hp - 14ac - +8 to hit - 8 damage - 1-2CR
Level 10 - 66hp - 16ac - +10 to hit - 18 damage - 4-5CR
Level 20 - 181hp - 17ac - +13 to hit - 30 damage - 9CR
Comments: Good statblock if you want a strong doggy companion, all around decent and its attacks also knock creatures prone.

Worg - 40ft. - Basic Rules - 5/5
Level 5 - 58hp - 13ac - +9 to hit - 11 damage - 2-3CR
Level 10 - 91hp - 14ac - +11 to hit - 24 damage - 4-5CR
Level 20 - 236hp - 15ac - +13 to hit - 36 damage - 10-11CR
Comments: This is what you get if you combine a wolf and a horse, good damage, good hp and knocks creatures prone. Can also learn to speak common, so you can talk with your (evil) furry friend

Wretched - 40ft. - Mordenkainen's Tome of Foes - 3/5
Level 5 - 22hp - 16ac - +7 to hit- 7 damage - 1CR
Level 10 - 35hp - 18ac - +9 to hit - 17 damage - 4CR
Level 20 - 100hp - 20ac - +13 to hit - 31 damage - 9CR
Comments: Incredibly strong natural ac, also has resistance to all melee (not just non-magical) while in dim light or darkness. Low hp due to starting with a negative con modifier, has a rather annoying to use attack that requires using movement to make extra attacks.

Zombie - 20ft. - Basic Rules - 2/5
Level 5 - 60hp - 8ac - +7 to hit - 5 damage - 1/2-2CR
Level 10 - 97hp - 9ac - +9 to hit - 13 damage - 3-5CR
Level 20 - 212hp - 9ac - +13 to hit - 22 damage - 8-10CR
Comments: Slow, low damage, low natural ac. Only thing going for it is the hp. Might not help your parties social standing either.

TL;DR

Here's the pet sidekicks grouped by rating.

5/5 - Giant Wasp, Needle Blight, Shadow, Steeder, Warhorse, Worg

4/5 - Ape, Cockatrice, Constrictor Snake, Cow, Dimetrodon, Draft Horse, Elk, Giant Centipede, Giant Goat, Giant Owl, Giant Poisonous Snake, Giant Wolf Spider, Hadrosaurus, Pseudodragon, Riding Horse, Rust Monster, Vine Blight, Wolf

3/5 - Axe Beak, Black Bear, Blink Dog, Boar, Crocodile, Darkmantle, Flumph, Flying Snake, Flying Sword, Giant Badger, Giant Bat, Giant Crab, Giant Frog, Giant Lizard, Kruthik, Magmin, Mastiff, Mule, Neogi Hatchling, Oblex Spawn, Panther, Pony, Psychic Gray Ooze, Pteranodon, Stench Kow, Vegepygmy, Velociraptor, The Wretched

2/5 - Ambush Drake, Blood Hawk, Boggle, Dretch, Giant Rat, Giant Weasel, Reef Shark, Slaad Tadpole, Twig Blight, Zombie

1/5 - Abyssal Wretch, Camel, Dolphin, Juvenile Mimic, Manes, Nupperibo, Star Spawn Grue

Thanks for reading. Place any comments about how many mistakes I made and how my opinions are stupid below.

Completely agree with your assessments other than possibly the boggle. Depending on how the DM interprets the portal it could set a portal on the floor with the exit 30ft straight up, other things can go through the portal if the boggle puts it through so the boggle could grapple a creature and drag it through it's portal, after falling far enough for max damage the boggle could just let go of the enemy and the player's main character could cast featherfall on the boggle leaving the target to take 20d6 fall damage. Also, the boggle using it's portal would count as being next to the enemy to work with flanking but the the enemy couldn't reach through the portal to hit the boggle.

A pixie would probably be more broken than anything, it has greater invisibility, flight, magic resistance, and could cast polymorph at level 1.

I have a player who's had a mastiff for a long time. I think it's time we just made the mastiff a side kick but when would it "level up"? Just at the 5, 10, and 20? For example, the PC is now 7th level. Would the mastiff use the level 5 template (Level 5 - 33hp - 12ac - +7 to hit - 5 damage - 1/2-1CR) until the PC turns 10th level?

In Tasha's the sidekick is a lot more powerful. I like this description for animal/pets better but I'm not sure I get how you arrived at the bonuses.

The bonuses, at least for the level 5 mastiff, are based on assuming the first level of Sidekick is gained (the mastiff gains 5 hit dice, not 4, for being level 5). 

I made you the L5 version as a homebrew, just to show you. I made some assumptions OP didn't cover to fill out the options I had to select, and I didn't buy the mastiff any barding.

Quote from PasadenaVic >>

I have a player who's had a mastiff for a long time. I think it's time we just made the mastiff a side kick but when would it "level up"? Just at the 5, 10, and 20? For example, the PC is now 7th level. Would the mastiff use the level 5 template (Level 5 - 33hp - 12ac - +7 to hit - 5 damage - 1/2-1CR) until the PC turns 10th level?

In Tasha's the sidekick is a lot more powerful. I like this description for animal/pets better but I'm not sure I get how you arrived at the bonuses.

 Is that a find steed mastiff. Because I wouldn’t upgrade that. If it’s just one they got from the shelter or whatever then a sidekick can make sense. Using Tasha’s rules, it levels up whenever the party does.

Thanks. I think I get it. I made it an "expert" because the PC purposefully has been keeping it OUT of combat for 7 levels (it would seem weird to make it a warrior now). This may end up giving it just extra hit points and defensive ability (e.g., evasion) but we'll see what the PC does with it...

Quote from PasadenaVic >>

Thanks. I think I get it. I made it an "expert" because the PC purposefully has been keeping it OUT of combat for 7 levels (it would seem weird to make it a warrior now). This may end up giving it just extra hit points and defensive ability (e.g., evasion) but we'll see what the PC does with it...

 You're the DM, so you can do whatever you want, but mastiffs can't legally be Experts - Experts have to be able to speak a language - so I assume OP ignored the corner case of using Awaken to make the dog capable of being an Expert.

Oh! I didn't see the language requirement. Thanks!

Quote from 11111Bob11111 >>

Completely agree with your assessments other than possibly the boggle. Depending on how the DM interprets the portal it could set a portal on the floor with the exit 30ft straight up, other things can go through the portal if the boggle puts it through so the boggle could grapple a creature and drag it through it's portal, after falling far enough for max damage the boggle could just let go of the enemy and the player's main character could cast featherfall on the boggle leaving the target to take 20d6 fall damage. Also, the boggle using it's portal would count as being next to the enemy to work with flanking but the the enemy couldn't reach through the portal to hit the boggle.

 What are you talking about?

30 feet is only 3d6 damage.

Quote from Dragonslayer9 >>

Quote from 11111Bob11111 >>

Completely agree with your assessments other than possibly the boggle. Depending on how the DM interprets the portal it could set a portal on the floor with the exit 30ft straight up, other things can go through the portal if the boggle puts it through so the boggle could grapple a creature and drag it through it's portal, after falling far enough for max damage the boggle could just let go of the enemy and the player's main character could cast featherfall on the boggle leaving the target to take 20d6 fall damage. Also, the boggle using it's portal would count as being next to the enemy to work with flanking but the the enemy couldn't reach through the portal to hit the boggle.

 What are you talking about?

30 feet is only 3d6 damage.

 Responding to someone from 9 months ago is... unlikely to be productive.

Quote from quindraco >>

Quote from Dragonslayer9 >>

Quote from 11111Bob11111 >>

Completely agree with your assessments other than possibly the boggle. Depending on how the DM interprets the portal it could set a portal on the floor with the exit 30ft straight up, other things can go through the portal if the boggle puts it through so the boggle could grapple a creature and drag it through it's portal, after falling far enough for max damage the boggle could just let go of the enemy and the player's main character could cast featherfall on the boggle leaving the target to take 20d6 fall damage. Also, the boggle using it's portal would count as being next to the enemy to work with flanking but the the enemy couldn't reach through the portal to hit the boggle.

 What are you talking about?

30 feet is only 3d6 damage.

 Responding to someone from 9 months ago is... unlikely to be productive.

 True.

I wonder, did he account for milking of poisonous pets?  Some people go looking for poisonous pets to obtain a free supply of poison for their own weapon attacks.

I think speak with dead, speak with animals or tongues spell would solve the language problem. Casting the spells for initial conversation with potential sidekick, then a collar or other item magically imbued with the spell to enable it to always speak. This should be a feasible solution for any potential sidekick. Shadows being undead, speak with undead + shadow ring or something similar.