White mare's daughter Epona Series, Book 1

In a time before history, in a land of legend, where wild horsemen wage endless war across a sea of grass, the last descendant of the White Mare's servants follows a sacred vision beyond the edge of the world. There she finds a land ruled by women, whose people have no word for war. But the horsemen are coming. The White Mare's daughter must try to stop them, and both save and change the world.