The Giant Horse of Oz (1928) is the twenty-second in the series of Oz books created by L. Frank Baum and his successors. The tiny kingdom of the Ozure Isles, perched on five islands in Lake Orizon, surrounded by high mountains in a remote region of Munchkin Land, has little contact with the outside world—of Oz. The evil witch Mombi has turned her malice in the Ozure direction. After kidnapping Queen Orin, Mombi left a fire-breathing lake monster named Quiberon in Lake Orizon to keep the natives prisoner. Even after Mombi was vanquished, Quiberon remains. Quiberon orders the Ozurites to kidnap a mortal maiden to keep him company. Since Oz is a fairyland, the only mortal maidens are three American girls living in the Emerald City; Dorothy Betsy, and Trot. Two Ozurites respond to the crisis. Philador seeks the aid of Tattypoo, the Good Witch of the North. Akbad, the Ozure Isles soothsayer, steals a pair of magic wings, flies to the Emerald City, and kidnaps Trot to serve Quiberon. In his search for Tattypoo, Prince Philador teams up with High Boy, a giant horse with telescoping legs. Various adventures ensue, in strange locations like Cave City, and with even stranger beings like the Roundabouties and Shutterfaces. Eventually, matters are sorted out satisfactorily: the Wizard turns Quiberon into a great bronze and silver statue, and the good Witch Tattypoo is revealed to be the missing and amnesiac Queen Orin. She is restored to her family and kingdom. Trot becomes a princess of the Ozure Isles, welcome in their Sapphire City whenever she chooses to visit.

Ruth Plumly Thompson was an American writer of children's stories, best known for writing many novels set in Oz, the fictional land of L. Frank Baum's classic children's novel The Wonderful Wizard of Oz and its sequels.