Once an animal spawns, it will never despawn. So it stays in the world until you kill it. But it also means, that once an animal is spawned, there will no more animals spawn at that location in the future (so if you kill all animals around your base, this place is free of animals, like in real). Currently they don't age yet, but we already prepared that. Diseases will come somewhen in the future, as well as breeding It's possible to keep animals in cages, but eventually this still needs some finetuning.