This changes all the time. An animal is a poor captive simply because we have a poor understanding of what environmental needs are important to it. As the leaders working with a particular species learn about what works and what doesn't, this information eventually becomes readily available.

I would refrain from the word pet, as a lot of good captives are not kept for pets, but for other reasons. In fact they would make terrible "pets". Venomous snakes are a prime example.

Ian