The three 11-year-olds, two girls and a boy, have a case to investigate. An extra pony is in the barn with the other baseball playing ponies. Where did it come from? Why is it there? The three find out it is of a rare and endangered breed. And learn why it's there.Suggest reading the series in order: 1. The Mystery at the Stables2. The Mystery at the Rodeo Ranch3. The Mystery of the Silver Saddle4. The Mystery of Sable Island5. The Mystery at the Horse Auction