"A New Horse" is the fourteenth story included in the book Scary Stories to Tell in the Dark.
There are two farmers. They share a room together in a barn. One sleeps by the door and one sleeps in the back. The one who sleeps by the door always has problems in the night. He says a witch, who lives with her husband not far from the barn, had turned him into a horse and rode him around. The one who usually sleeps in the back decides to sleep by the door instead. Sure enough, the witch comes and turns him into a horse with a magical saddle and rides him around, whipping him to make him go faster. Then he throws the saddle off and puts it on the witch, turning her into a horse. He gives her to her husband, in exchange for a better horse. When the man takes the saddle off and puts it away, the horse is gone and all he can see is his wife.