A filly is a young female horse, usually one that's under four years old. Once she turns four, she's called a mare. Think of it like the horse equivalent of a girl becoming a young woman.

Horse owners and breeders pay close attention to whether a young horse is a filly or a colt (a young male horse) because it affects how they train them and what roles they might play. In horse racing, fillies often compete in their own races, like the Kentucky Oaks, which is run the day before the famous Kentucky Derby. Sometimes an exceptional filly will race against colts and win, which creates excitement because fillies are typically smaller and lighter than colts.

You might hear someone describe a young, energetic girl as a filly, though this usage is old-fashioned and not very common anymore, and it can sound disrespectful.