This was a young adult novel I read in maybe 1995 or 1996, and I remember little of the story except that it was about a horse prince whose horse culture had a savior prophecy to defeat their ultimate enemies that were some other supernatural creatures, maybe? The savior would be revealed by a black (or maybe brown) coat and a white blaze on the horses' forelock.
The main character starts out as a bit of a hot-headed rebel who thinks he can defeat the talking horses's enemy, but then some misadventures happen that I can't remember, and then the next season when he sheds his winter coat, he realizes he has the markings of the prophesized savior and then the book ends, or he helps defeat their enemies, I can't recall. It might have been part of a series? I always thought the name of the book was Cimarron, or something like that, but that is a different book about horses so I suspect I was conflating the two.
I tried to find this with the details I remember, and one option came up as similar but it was about unicorns and I'm pretty sure this was about regular (but talking) horses, and I haven't been able to find a copy of that book (Birth of The Firebringer) to confirm.