We have horses and sheep in the same pasture at the same time. No problem. Actually, I read in one of my horse books that sheep are good companions to horses because their bodies kill parasites and such that can kill horses. So, actually, they help keep horses healthy
So there's no problem with horses and sheep sharing a pasture
One thing about having a young horse in with sheep at the same time, is this might result, lol!
ETA: I've owned horses for almost four years now, all the time having sheep in the pasture as well, and we haven't had a single problem