Quote from grommen
Kill every cow, chicken, wolf, pig (anything that does not go Naaaaaee).
And keep killing them.
You'll get horses.
Their appears to be a limited amount of mobs that can spawn on a planes. The other farm animals are taking up the space for the horses to spawn. It also looks like untamed horses despawn at the moment. Not sure if that is true or not, but I had some vanish over night on me. The tame ones appear to stay.
ok the plains biome is really big and i killed a heap of animals and now nothing seems top spawn anymore i can't find any living thing in the plains except myself