A filly is a young female horse that is less than 4 years old and has not yet been bred. In horse racing, filly races are separate from colt or gelding races, allowing female horses to compete against each other.
Filly races are often seen as an opportunity for fillies to prove their worth and compete against their gender, rather than facing the typically larger and stronger colts. Some of the most famous fillies in horse racing history include Zenyatta, Ruffian, and Rachel Alexandra, who have all achieved great success on the track.