Filly a young female horse too young to be called a mare. There are several specific definitions in use.
- In most cases filly is a female horse under the age of four years old.
- In some nations, the world of horse racing sets the cutoff age for fillies as younger than five years.
On June 9, 2007, Rags to Riches became the first filly to win the Belmont Stakes since 1905.