There are seven degrees of animal rarity:

B - Basic (gray)
C - Common (green)
U - Uncommon (blue)
R - Rare (purple)
E - Exotic (orange)
M - Mythic (gold)
A - Amazing (pearl)

Basic animals can be bought in the shop.

Other animals can be unlocked by experimenting in the Laboratory or purchased in the Curiosity Shop and via special offers or events.