There were probably several different variations of the ancestral wild (never domesticated) horses, before the dawn of the age of civilization. But they would have been either domesticated, or killed off by extensive hunting, as horseflesh may have been considered a delicacy in the early days of dawning of culture, and certainly, horsehide is a much coveted form of leather.
The horse genome has not changed terribly much, as horses gone feral have been able to adapt to a wide range of conditions and even climates, from desert to forest cover. The war horses and draft horses of Europe (Belgians, Clydesdales, Percherons and others) were descended from a variant found in forests, the “forest horse”, while the western mustang is probably descended from desert horses known as Arabians.
Selective breeding created the horse as we know it, AFAIK. I think their ancestors were much too small to be useful for much . . .