Reilly & Lee, Chicago, 1963, illus Constance McMillan, 82 pp.
When Lynn Morgan goes to take care of the family’s ponies in the summer pasture, he is warned not to talk to Owen Kern because the two families are feuding, and to take very good care of the ponies so that their Druid King will want to buy them.
Helen B Walters was born in Porterage, Wisconsin, and had two children. She wrote several non fiction children’s books on scientists and historical figures, and as far as I know, just the one horse book.
Finding the book:Ponies for a King is reasonably easy to find in the USA, and not expensive.