The Hero of Abarxia — Deborah J. Ross

The prince returns triumphant from a decisive battle, riding a superb, shimmering golden stallion, the prized steed of his vanquished foe. Even as the kingdom rejoices in the prince's victory, all is not well with the horse. It is rapidly wasting away from a strange ailment. The aged king seeks a magical procedure that will allow him to temporarily exchange minds with the horse and learn the reason for its decline. A short story.