The Miraculous Birth. Light Horse, Dark Horse, #1

Long ago, the world of Equus and its residents- the horses, donkeys, zebras, and ponies- were made perfect and good by the divine Great Horse. Until Abaddon, the evil Dark Horse, tricked the equines into rejecting their loyalty to their Creator. As a result, a curse fell upon the equines, dooming them to eternal enslavement. But there is hope. The Great Horse chooses two unlikely mustangs: a filly name Sierra and a young stallion name Shadow.
They are to raise the Light Horse, who is destined to break the curse and rule as the eternal King of Equus. But the Dark Horse will do everything in his power to prevent the birth. As Sierra and Shadow face their greatest fears and deepest desires, they'll discover that their choice will change their lives and the lives of many others forever.