I may be wrong, but I think rabbits will still spawn if we use this world. Horses are different and spawn as the chunks generate. Rabbits use the same spawning mechanics as pigs, sheep, etc. Which means they spawn on grass randomly. And the only other thing is underwater temples, which will not be able to spawn. (I've heard updated generation occurs 1,000+ blocks from where anyone has generated chunks, but I'm not sure). Maybe we should give the world a chance before resetting everything.