Ponyville Mysteries: Riddle of the Rusty Horseshoe

My Little Pony: Ponyville Mysteries: The Tail of the Timberwolf 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.

Lilymoon's first sleepover
The story begins with "The Pony" from the previous books moving through the bogle's nest in the Everfree Forest and as getting as close as they can to the Livewood before alerting its patrol of timberwolves. As "The Pony" feels the magic pouring out of the Livewood, they think of a way to keep the Cutie Mark Crusaders from getting closer to the Livewood's secrets.

At Lilymoon's house, she and the Crusaders play games at Lilymoon's first slumber party. Lilymoon doesn't understand some of the games being played, so Apple Bloom suggests a different activity, and Lilymoon decides on telling ghost stories. Scootaloo notices Apple Bloom's overeagerness to have fun with Lilymoon and feels she is paying more attention to Lilymoon lately than her or Sweetie Belle. Just as Lilymoon starts to tell her story, Scootaloo interrupts, knowing that Sweetie Belle was still sensitive to scary stuff after their recent encounter with a timberwolf.

Scootaloo decides to tell the tale of the Olden Pony—a story that Sweetie Belle already knows—but Apple Bloom protests, saying Lilymoon wants to go first. The two fillies get into a heated argument, and Apple Bloom blurts out that the story of the Olden Pony gave Scootaloo nightmares the first time she heard it. Upset that Apple Bloom would reveal such a secret to Lilymoon, Scootaloo angrily leaves the room to get a snack in the kitchen.

Return of the Olden Pony
On her way to the kitchen, Scootaloo hears something whisper to her, and she dashes out the back door in terror and into Lilymoon's family greenhouse, knocking over a large potted plant in the process. The thing that whispered to Scootaloo is revealed to be Lilymoon's older sister Ambermoon. Scootaloo still doesn't trust Ambermoon after the werepony incident in the previous book was traced back to her.

After cleaning up the potted plant mess, Scootaloo asks why Ambermoon is following her around in the dark, and after some initial grumpiness, Ambermoon says she overheard her arguing with Apple Bloom and wanted to make sure she was okay. While getting a snack in the kitchen, Scootaloo hears another voice whispering to her and slowly getting louder. She races into the house foyer and sees the Olden Pony herself standing at the top of the stairs. As she moans about her rusty horseshoe, Scootaloo is overcome with fear and gallops out the front door toward home.

The next morning, Scootaloo is greeted by her aunts Holiday and Lofty. When asked why she returned home so suddenly from Lilymoon's sleepover, Scootaloo says she wasn't feeling well. Apple Bloom, Sweetie Belle, Lilymoon, and Ambermoon arrive to check up on her, and when Scootaloo claims to have seen the Olden Pony, her friends are skeptical. Dismayed that they don't believe her, Scootaloo declines their invitation to Sugarcube Corner and sets out to prove what she saw was real.