You have to wait. I have pigs and chickens and cows, and one lone sheep...
I don't think they HAVE to spawn on grass, even though it says so on the wiki. How sheep spawn near my mobgrinder, IDK. They wouldn't have jumped there, because it's at least 5 blocks, and animals tend to avoid damage.
Then lighting struck my chickens and cows and pigs...
EDIT: I also found untamed wolves. When I went back to get bones, they despawned (The grassy mob spawning 'park' was two-three chunks away)
Best way to get them there would be to explore elsewhere till you find what you want, then use wheat to lure them to where you want them. If you want more, find at least 2 and make them breed using wheat. Other than that, use a mod to spawn the animals you want.
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You need a 6x7 well lit patch of grass 24 blocks away from you. If you be patient, they will spawn after a looong while. During the wait, I suggest you mine some cobblestone in your generator or whatever.