I think it is either Epona or Rhiannon; both goddesses of horses.
I'm pretty sure it is Dun Lady's Jess, a horse turned into a human. It took me awhile to track down confirmation, but that is the Baen cover to Changespell, written by Doranna Durgin. I don't think I read all of those books, so I'm not sure if only the one mare gets turned into a woman, or if there was another.Baen had Elmore do more than one cover for Durgin.
Sooo, is it a blessing . . . or a curse? LOL