First off, if you are playing Adventurers League (DDAL), playing by DDAL-like rules, playing "RAW", or have a predetermined opinion and/or are unwilling to try new things, please don't bother engaging in this thread. I'm hoping this thread will spark some helpful and constructive conversation to improve the enjoyment of the game for those of us that want to play a PHB beast master ranger.

Dungeon Master's! On pages 273 to 283 of the Dungeon Master's Guide we are given directions on how to adjust a "monster" (NPC) stat block, create a "monster" (NPC) from scratch, or add class levels to a "monster" (NPC). The 3 requirements for something to be a ranger's companion are be of size medium or smaller, be of challenge rating 1/4 or lower, and have the type "beast". This, unlike spells like find familiar and find steed that tell specifically what creatures can be used with the spell, just gives you some parameters to follow. The folks at WotC have taken this idea to heart with the new book from them coming this November, Tasha's Cauldron of Everything.  I personally don't think the beast master in the PHB is a bad subclass. However, I don't think the ranger class in the PHB is a bad class, AND I interpret most of the ranger abilities in what I consider RAW yet forgiving way. But that's a different conversation. Using those DMG pages and the PHB ranger abilities, you as the dungeon master can, by yourself or with your player, make some small adjustments to the companions (using a stat block from an official book as starting point) that exist and bring your ranger from "ok" to "a shining star"!

The wolf, giant poisonous snake, and flying snake are considered by many (not an exclusive list, but certainly top picks) to be the "beast" companions to pick, for combat mostly. (Although I don't believe combat is the primary focus of the beats master ranger, combat is most of what the internet argues about, so that will be our focus.) One thing they have in common is a high ability to hit things in combat, either by pact tactics or an insane attack ability modifier. A quick read of the pages mentioned above will show you the numbers, parameters, and method for adjusting a stat block and finding the new CR. There are several calculators on the internet that will do this work for you as well. This and this are pretty good. Take the animal your player wants, and use these rules given to you as the dungeon master to make it awesome! Bump it's attack stat up one (this alone does a lot for the subclass in combat). Give it proficiency in two (the good ones like dexterity, constitution, and wisdom) saving throws (proficiency in 0-2 saving throws doesn't effect the creature CR) (I suggest two of the saving throws that the ranger doesn't have, like constitution and wisdom, to diversify). Give them another or several skill proficiencies, or even better, expertise in one or two skills (none of this effects CR)! Give them an effect with the attack that hits as a rider (the giant frog, crab, and panther are great examples of these). Give them a unique reactions (something they get to do for free) like the last part of the polearm master feat, or being able to move up to it's speed if attacked. Use your imagination and have fun crating! Think of these abilities as something like the battle master maneuvers. Maybe the hawk (the normal kind) grants advantage to the next attack against a target as part of its attack (something like a "distracting strike").

I always give companions death saving throws. You're being a jerk if you don't. I also run the companion's turn and ranger's turn as one in initiative. Two turns, but overlapping, happening in tandem. I run them just like a controlled mount and rider, identical group of monsters controlled by the DM, animals pulling a chariot or cart, and the like. It transforms the companion from a clunky and "expensive familiar" into an active and direct extension of the ranger, which is what a subclass should do.

I’ve always favored monkeys. Baboon in 5E, I guess. But I have to say, the Giant Wolf Spider is a hoot of an animal companion.

I think the biggest issues with the Beast Master are:

1. Pet has very few hit points.  The pet has about the same number of hit points as a wizard with 10 constitution.  Considering in combat they're mostly required to be in melee range, that's real bad.  A young dragon's breath weapon will likely one shot them all the way until well after you'd see a young dragon.  A CR 8 Young Bronze Dragon's breath attack does 10d10 lightning damage with a CR 15 dex save for half.  That's an average of 55 damage on a failed save, or 27 on a successful one.  A level 8 Ranger's pet has 32 hit points by that point so there is a good chance that even on passing the save it'll go down.  At 15 the Ranger can cast Absorb elements and have it help protect the pet but that's a long time to go without it.  
2. You have to give up an attack to allow it to attack.  Generally speaking, your attacks are going to be stronger than the pet's attack until 11 which means it won't be don't much of anything until level 7 when it can start taking the help action using your bonus action.  You'll likely want to take a good feat like GWM or Sharpshooter which even further makes it less good.

So how do we solve these two major problems? 

1. I think giving it evasion is a good start to solving number 1.  It won't help against all AoE damage but it will help against a lot of it.  It won't be able to stand up against attacks still but hopefully the DM is kind and doesn't focus the pet.
2. I like where you're at thinking about giving the pet more to do than attack.  The most successful pets have flyby in my opinion so that it doesn't have to stay there risking getting blown up.  Maybe our evasive pet can also have flyby or something similar to that.

Giving it something to do on its reaction could be good.  Giving it something defensive to use as its reaction would be preferred.  Maybe give it uncanny dodge?  

Those are my initial thoughts anyway.

Any custom pet idea needs to resolve these issues or at least off set them.  I for one think the ranger is a very strong class.  Customizing the animal companion is a good idea to try to strengthen the subclass.  

Quote from 01011000Lehrer >>

I think the biggest issues with the Beast Master are:

1. Pet has very few hit points.  The pet has about the same number of hit points as a wizard with 10 constitution.  Considering in combat they're mostly required to be in melee range, that's real bad.  A young dragon's breath weapon will likely one shot them all the way until well after you'd see a young dragon.  A CR 8 Young Bronze Dragon's breath attack does 10d10 lightning damage with a CR 15 dex save for half.  That's an average of 55 damage on a failed save, or 27 on a successful one.  A level 8 Ranger's pet has 32 hit points by that point so there is a good chance that even on passing the save it'll go down.  At 15 the Ranger can cast Absorb elements and have it help protect the pet but that's a long time to go without it.  
2. You have to give up an attack to allow it to attack.  Generally speaking, your attacks are going to be stronger than the pet's attack until 11 which means it won't be don't much of anything until level 7 when it can start taking the help action using your bonus action.  You'll likely want to take a good feat like GWM or Sharpshooter which even further makes it less good.

 

So how do we solve these two major problems? 

1. I think giving it evasion is a good start to solving number 1.  It won't help against all AoE damage but it will help against a lot of it.  It won't be able to stand up against attacks still but hopefully the DM is kind and doesn't focus the pet.
2. I like where you're at thinking about giving the pet more to do than attack.  The most successful pets have flyby in my opinion so that it doesn't have to stay there risking getting blown up.  Maybe our evasive pet can also have flyby or something similar to that.

Giving it something to do on its reaction could be good.  Giving it something defensive to use as its reaction would be preferred.  Maybe give it uncanny dodge?  

 

Those are my initial thoughts anyway.

 

 

Any custom pet idea needs to resolve these issues or at least off set them.  I for one think the ranger is a very strong class.  Customizing the animal companion is a good idea to try to strengthen the subclass.  

 

 The big AoE situation is mentioned often. I guess I’m not too worried about it. You can always just get a replacement beast. That AoE is going to wipe out steeds, familiars, the squishy characters are going to fair badly also, and everyone will take a lot of damage. So my thoughts in response are, one, why did the dragon get the drop on the party to begin with? Wouldn’t the ranger be the very one to not let this happen? Between primeval awareness, tracking, stealth, perception (many of the beasts have bananas good passive perception), and if dragons are a favored enemy of the ranger this would be the very thing to avoid with that ability by recalling information about the dragon and putting together a plan to avoid it. Two, if this hits the party many could die, wizard included. At higher levels the ranger has share spells and can learn absorb elements, but at this level everyone is hit hard and there is going to be a LOT to discuss after the fight, assuming they win or get away. Third, in the little blurb about death saving throws in the PHB it talks about NPCs getting to make them or not. It lists some thoughts on the subject and the beast companion definitely falls into that category of “should make them”, in my opinion. That alone would eliminate this concern insofar as the “pet” dying. The wizard and the Fido can be badly hit, maybe unconscious, but alive.

For the attack thought, I don’t think the beast is there to replace the ranger’s attacks on a one for one basis. It’s like adding to the attack options of the ranger versus replacing them, like ha e multiple weapons, or several attack cantrips. Most decent beasts (either a d6 or higher damage die or +3 ability modifier) do the same damage as a ranger with a bow when hunter’s mark is off. There is a whole bunch of folks that complain about hunter’s mark and losing it because of concentration (also not a big deal to me). When this happens the beasts attack are doing the same damage but many with an additional effect like knocking prone, grappling, or poison, and then some of them have situations where they deal even more damage if the situation is right (charge or extra attack against a prone target). If the ranger is melee they could make a ranged attack with their blood hawk. If the ranger is ranged they might have a wolf with a higher armor class and advantage to hit targets that can block, guard, how with additional AoO, knock targets prone, and if nothing else take some hits that would otherwise be hitting the other party members. With barding, even cheap barding, a wolf can have an armor class of 19 or higher. Crabs and snakes are off the charts! Also, because most good beasts have the same damage output as a ranger’s attack without hunter’s mark, the total damage output stays higher when using a spell that isn’t hunter’s mark. Hail of thorns is concentration so the ranger “loses” the hunter’s mark damage when using it. By choosing to use the beast’s attack at this point the damage output stays higher than it would without a beast. At levels 11 and higher the average damage output is even higher which means the net “loss” of not having hunter’s mark is almost nonexistent. Fighters (battle master, specifically) get to push or trip with an added d8, while a beast master gets to with a beast. The ranger might miss out on a d6 or two of damage, but it can be done all the time, from two points on the battle field, with a little added “mini tank” with extra hit points, and even better damage at higher levels. The beasts that don’t have auto grapple effects when they attack are still taking the attack action so they can grapple. Several of the really great ranger damage spells are small AoE. The bigger beasts can grapple and position a enemy or two to setup these great spells.

Im a big fan of making the beast what it needs to be while staying inside the RAW confines. RAW in the DMG teaches you how, as a DM to tinker with NPCs, which I think is fun and do anyway even on a cultist or alone devil. With only the few requirements for a beast companion the amount of wiggle room for creativity to make or add to a existing beast is amazing! You mentioned reaction options. Adding the last part of the pole-arm master feat to the beast is loads of fun and makes them an even better “guard dog”. I would even trade pact tactics out and replace it with a reaction “creatures trigger an AoO when they enter the beast’s reach.”

Personally I turn off passive perception when I’m combat. For a couple of reasons, including the rouge’s reliable talent takes a hit otherwise and dragons have a legendary action to make a perception check. Anyway, since beasts have such good perception even giving them a bonus action to “make a perception check” would be, at least, fun and thematic, and at beast amazing! Amazing, perhaps, because they end up being the only creature to spot the dragon heading your way to blast your with its breath weapon. 🙂

Okay, so, if we're going to devise improvements for the Animal Companion then we need to address its weaknesses and its strengths.

Strengths

1. They add the ranger's proficiency bonus to AC, attack rolls, damage rolls, and skills they're proficient in. These are all solid buffs; bringing the beast to roughly CR 1 at 3rd-level and CR 5-6 at 20th. This puts them on par with some of the beefer elementals in the Monster Manual, and the best Wild Shape options available to druids.
2. They gain magical attacks at Ranger 7 and Multiattack at Ranger 11; granting the ranger 3+ attacks per turn.
3. They can equip barding to improve their AC and even attune to magic items.
4. The Share Spells feature at Ranger 15 is fantastic, but it comes at so late a level it may not see much use. That said, it works with (but not only)...
   1. Absorb Elements to withstand certain energy attacks
   2. Conjure Barrage for some impressive damage
   3. Ensnaring Strike to Restrain multiple foes
   4. Pass Without Trace so the party can stealthily split into two groups for a pincer move
   5. Zephyr Strike for a little extra damage and mobility

Weaknesses

1. Their hit points are either their normal maximum or 4 times the ranger's level. This can start strong, tapering off as they gain in levels, but they don't get additional hit dice to recover during a short rest.
2. If they have a rider effect, such as the wolf's bite/trip or the giant crab's claw/grapple, these do not improve.
3. They do not add the ranger's proficiency bonus to saving throws they're proficient in because no eligible beasts are proficient in any saving throws.
4. According to the RAW, rangers cannot use two-weapon fighting while commanding their Animal Companion to attack.

So, let's fix those weaknesses.

1. They have a number of hit dice equal to the ranger's level. This will bolster most Animal Companions, save for the small and tiny ones. For them, keep the 4/level the PHB describes.
2. Either add the ranger's proficiency bonus to their rider effects or use the ranger's spell save DC in its place. The former allows for a respectable DC of 17-18 (depending on the chosen beast) without investing too much in Wisdom. The latter incentivizes the ranger to improve their Wisdom to 18 or higher.
3. Add the ranger's proficiency bonus to all of their saving throws, including Death Saving Throws. This last one, I feel, was an oversight in the PHB that would have been fixed by simply adding a comma. But because so many writers and editors dislike the Oxford comma, it got left out. This, unfortunately, leaves the sentence in question open to interpretation.
4. Allow two-weapon fighting, if you like. Personally, I like the restriction as it disincentivizes reliance on Hunter's Mark.