Level 9 animals are animals which, like regular animals are not the base animal, or the normal species of that animal, but can only be found in the stampede through normal play, provided that the player upgrades the respective habitat to level 9. All level 9 animals give animal mastery XP, and are classified under tier 4.
The Level 9 classification only applies to Sky Zoo animals, due to the way that animals are released in the Space Zoo. In the Space Zoo, 'level 9' animals are unlocked by upgrading a habitat to level 9, the same way as unlocking other regular animals, just at a higher level. This means that regular and 'level 9' animals are the same in the Space Zoo, meaning there are no level 9 animals in the Space Zoo.