I'm sorta disappointed with the animal system at its current state, admittedly - thought it'd be a lot more useful and able to be fine-tuned.
For one, creatures appointed NEED to have a pet role to actually show up - war trainable, mount, wagon puller, etc. Even if they have the proper class token, they won't show up unless they have a role token or you use one of the overrides - which defeats the purpose of having non-pet animals added. Don't exactly want to see a large demon riding into a siege on a tiny imp. Or imp wagon pullers. Or well, any other "pet" role, honestly. Was hoping we could use this to have innate intelligent wilderness creature populations in civs that would be playable - even if you do give them the overrides they won't be playable, though.
The ANIMAL_ALWAYS_PRESENT token doesn't work for this either - they just don't get spawned.
Secondly, the populations of said creatures seem affected by the role as well. Small example:
The Shredders, Sky Golems, and Hobgoblins are all mounts, and so seem to have much higher populations than the Harvest Golems, which are only pets. Having 2 of them for the entire civ though? Quite ridiculously low.
I hope this gets expanded a lot more, since there's great potential here.