My Little Pony: Ponyville Mysteries: Riddle of the Rusty Horseshoe by "Penumbra Quill" (collective pen name of Michael Vogel and Nicole Dubuc) is the twenty-seventh book in a series of chapter books published by Little, Brown and Company and the third entry in the "Ponyville Mysteries" series. In this book, the Cutie Mark Crusaders must face their deepest fears when the Olden Pony manifests in the real world.