Your Constitution increases by 1

Thanks to your animal side, you can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.

A spirit shifter's alignment depends on the pack they choose at ten years old. But most are on the neutral side

In your natural form you are the same size and shape as a human. Standing anywhere from 5 feet to over 6 feet tall. Your size is Medium.

Your base walking speed is 30 feet.

You gain proficiency in the Animal Handling skill .

Thanks to you animal side, you are proficient in the Perception Skill.

Starting at 3rd level you can change into your chosen animal once per long rest.
In animal form use your stats and hp. When you Hit Points fall to 0 you return to your natural form and are unconscious.

At 9th,17th levels you get one additional charge.
At 20th level you may change at will.

Your animal nature takes over. On your turn, you can make one additional attack with your natural weapon.
The number of times you can do this, per long rest, is equal to your Dex mod.

You have a natural weapon appropriate to your animal type. Claw/Hoof(slashing), Bite/Horn(piercing), Tail/body slam(bludgeoning).
What you have will be stated in the “Animal Boon” list below or granted at DM discretion.
Your Natural Weapon does 1d4 damage

You can speak, read, and write Common and one other language of your choice.

Shifters get a set of animal-specific boons that apply in either form.
Some animals are given weaknesses for balancing purposes.
These are NOT subraces.

Feline

+1 to CHA
Gain Produce Fire cantrip
Stealthy: Gain Advantage on Stealth rolls
Fur Coat: You have resistance to the cold
Natural Weapon(s): Claw, Bite

Canine

+1 to CHA
Gain Mold Earth cantrip
Growl: Gain Proficiency in Intimidation
Fur Coat: You have resistance to the cold
Natural Weapon(s): Claw, Bite

Avian

+1 WIS
Gain Touch Wind cantrip
Like Flying But It's Falling: Gain Feather Fall
Weakness: Bird-bones: When an attack lands, roll a d20. On a 1 you break a bone and are unable to move. Your condition counts as Restrained. The effect last 1d6 turns or until you can be healed.
Natural Weapon(s): Claw, Bite

Ursine, Porcine, Bovine

Thick Skinned: Additional +1 to CON. +1 to AC
Gain Mold Earth cantrip
Multiattack: Make two attacks: one with your bite and one with your claws.
Natural Weapon(s): Claw, Bite, Body Slam

Cervine, Equine

+1 CON OR DEX
Gain Mold Earth cantrip
Like The Wind: Increase speed to 40
Natural Weapon(s): Hoof, Bite, Body Slam

All aquatic animals
This is includes mammal, fish, or other

+2 to INT OR DEX
Gain Shape Water cantrip
Water breathing: Breath water as easily as air
Strong Swimmer: You have a swimming speed of 30 feet.
Resistance to cold damage
Weakness: Is it hot in here?: You have weakness against fire damage
Natural Weapon(s): Claw, Bite, Tail Slam, Body Slam as applicable

Insect and Arachnid

Poison Resistance
Float Like A Butterfly: Attack rolls against you are made at disadvantage
Sting Like A Bee: You produce a poison from blood, sweat, saliva or other means. Coat your weapons or use your bite to add 1d4 poison damage to your damage rolls.
Weakness: Rip The Wings: When an attack lands, roll a d20. On a 1 you fall prone and your condition counts as restrained for1d6 turns or until you can be healed.
Natural Weapon(s): Bite

Rodent, Lapine, and Mustalid

+2 to DEX
Gain Mold Earth cantrip
Nimble: Attack rolls against you are made at disadvantage
Stealthy: Gain Advantage of Stealth rolls
Natural Weapon(s): Claw, Bite

Reptile

Scale Armor: +2 to AC
Gain Produce Flame cantrip
Venom: Your saliva is venomous. Coat your weapons or use your bite to add 1d4 poison damage to your damage rolls.
Weakness: Soft Underbelly: When an attack lands, roll a d20. On a 1 you are now bleeding and loose 1d4 hp for 3 turns.
Natural Weapon(s): Claw, Bite

Special

If your animal type isn't listed, work with your DM and pick from the provided lists to best fit.
Or, work with your DM to figure out boons to fit your animal.
Magical animals such as Manticores and Unicorns can be spirit bonded. But you do not gain their abilities. Abilities are at DM discretion.
Suggested boons: +1 to a suitable stat or AC
One elemental, sensory-focused cantrip Like Druidcraft or Mold Earth
At least one suitable boon, such as advantage on on a roll or a poison ability
For additional balancing, add a damage weakness or an effect that incapacitates/restrains for a limited time.
Lastly, lay out what natural weapons they have: Claw/Hoof (slashing), Bite (piercing), Tail/body slam(bludgeoning).