Mystery of the Midnight Rider (Nancy Drew Diaries #3) by Carolyn Keene

Dear Diary,

WHO WOULD WANT TO HARM A horse? I kept asking myself that question while watching the beautiful jumpers at the annual River Heights Horse Show. The top riders from all over the country are here, and one of them, Payton Evans, is a close friend of Ned's family. She's been training for years, and Ned said she might even make the Olympic team. But just when Payton was suppoosed to compete in this show, everything horribly wrong: Her horse, Midnight, tested positive for drugs! What kind of person would ever give drugs to a horse? And why?