The female bloodline of modern domesticated horses shows extreme diversity... at least seventy-seven ancestral mares are required to account for the genetic variety in modern population. Meanwhile, the male aspect of modern horse DNA shows remarkable homogeneity. It is possible that just a single wild stallion was domesticated.
One male horse. Many female horses. Think about it.
Stallions are headstrong and violent... A relatively docile and controllable mare could be found at the bottom of the pecking order in many wild horse bands, but a relatively docile and controllable stallion was an unusual individual--and one that had little hope of reproducing in the wild... From the horse's perspective, humans were the only way he could get a girl.