In version 1.9pre2 animal spawns are mostly a once-only thing, meaning once a player kills a passive animal mob, they are mostly dead for good, or difficult to get back. This can be counteracted by breeding, which begins when two same animals fed with wheat are near each other. Future versions, based on screenshots, will have baby animals, and may reduce the speed of animal breeding, just so people cannot get a sheep farm going in seconds feeding fully-grown animals spawned from breeding.