Animal tiers are tiers not explicitly stated in game but are a distinguishing feature between rarity of animals. Tiers can be definitively distinguished by the breeding time in the zoo when a 'match' is found, the one exception to this being the Cape Buffalo and the Zebra, whose breeding times are unique. Other less definitive distinguishing factors include XP given, what class of animal it is, where it can be obtained from and the order in which it comes out in the zoo.
* Note: The order in which the animals come out of the shelter in the far left corner of each habitat was modified in version 1.7.0. One random animal will appear in each habitat to begin with, however when the habitat is tapped, all other animals will appear in the correct order.