Ruth Plumly Thompson 
Wishing Horse of Oz 

The Wishing Horse of Oz (1935) is the twenty-ninth in the series of Oz books created by L. Frank Baum and his successors, and the fifteenth written by Ruth Plumly Thompson. It was Illustrated by John R. Neill. This book marked the point at which Thompson had written more Oz books than Baum himself. This Oz mystery starts in the small, poor kingdom of Skampavia, where King Skamperoo wishes for a horse using enchanted emerald necklaces. When Chalk, a talking Horse from Oz, falls from the sky, Skamperoo decides the emeralds must be from the Emerald City, and decides to conquer all of Oz. He magically causes all the residents of Oz to forget their rightful rulers and accept him as their emperor instead. Only Dorothy and Pigasus, the flying pig, are able to remember Princess Ozma, the true ruler of Oz, and together they set out to rescue her. The mystery in this story is how to make the necklaces grant wishes; only the horse Chalk knows how to do this.