Dark Horse — Mary H. Herbert

After her entire clan is massacred, a young woman assumes her brother's identity and becomes a warrior--all to exact revenge upon the chieftain who ordered her family slain. But the chieftain, Lord Medb, has resurrected the forbidden art of sorcery and plans to destroy all who oppose him in this dark ages fantasy world.

With the help of an intelligent, magical horse, the young warrior-woman goes against tradition and law to learn sorcery, all in the hope of thwarting Medb's evil plans of conquest.