I definitely wouldnt go for "zoo" animals.
If they are just objects it'd be boring, and if the have a kind of idle animation it'd be looking weird. About "classic zoo" animals that attack, I think it'd be a waste of your time and ressources as these are not widely usable.
So if you are about to design an animal pack I'd go for everything that would classify as a "LAB" animal.
Small animals like mice or guinepigs - or even birds- don't look to weird with a kind of "static" animation and larger typical lab animals (it hurts but thats reality) like cats, dogs or monkeys are eligible to attack the player. It could include larger predators like tigers or even reptiles
Since experimental labs are quite a common feature in FPSC games so you'd have better chances people would be willing to spend money on your pack.
In case you find my grammar and spelling weird ---> native German speaker ^^