his is a book-length fairy tale I found in our public library in the mid-1960’s. it was an old-looking book then, and was not specifically a children’s book, more in the style of Lord Dunsany or William Morris. The book concerns a human boy who is stolen by the fairies when a baby. The child becomes a favorite of the fairy king’s daughter, grows to adulthood in fairyland, and becomes a fairy knight.
I recall two scenes in particular. In one, the fairy princess has beseeched her father to take particular care of the changeling child. The fairy king commands the land to treat him well, in a spell beginning “Grass, be soft on his unused feet,” or words to that effect.
In the second, the boy has grown up. A fairy knight has to conjure/summon a horse from a mystic gulf by memorizing and then chanting the summoning. He then turns and walks out of the cave, not looking back until the horse he has called follows him out. How well he performs the summoning determines how strong and beautiful the horse will be.