There are seven groups of animals, and each group has three animals.
You can give animals to your Chao to make it stronger and give it different personalities and looks. When you give a Chao an animal, it will hug it, and attain its looks and skills. The animal will disappear after you give it to a Chao.

Animals are found hidden in the levels, usually in locked cages that you have to destroy or in pipes scattered around the stage (that you have to whistle at). You can also find them in robots created by Dr. Eggman such as the bomb throwing monkeys, or the robots that look like E-102 Gamma.

When you give your Chao an animal, it will not only gain its looks and skills, but also an action. These are the Animals that are in each group and what action it gives your Chao.

Yellow Group: Swimming

Penguin - Slides About on Belly
Seal - Rolls Around
Otter - Does the Backstroke when Swimming

Purple Group: Flying

Parrot - Sings
Peacock - Struts About
Condor - Flaps About

Green Group: Running

Warthog - Dashes
Rabbit - Hops About
Cheetah - Licks its Face

Red Group: Power

Tiger - Sharpens its Nails
Bear - Roars
Gorilla - Beats its Chest

Blue Group: Random

Racoon - Spins Around
Skunk - Farts
Sheep - Does Forward Somersaults

Gold Group: Mythical

Unicorn - Bucks its Feet
Dragon - Breathes Fire
Phoenix - Wags its Tail

Black Group: Ghost

Bat - Makes your Chao's legs dissapear
Skeleton Dog - Removes Animal Features from Chao, Allows Hats to be Worn
Half Fish - Turns the Chao Ball into a Flame