A filly is a young femalehorse too young to be called a mare. There are two specific definitions in use.
In most cases filly is a female horse under the age of four years.
In some nations, such as the United Kingdom and the United States, the world of horse racing sets the cutoff age for fillies as younger than five years.
The equivalent term for a male is a colt. When horses of either sex are less than one year of age, they are referred to as foals. Horses between one and two years of age may also be called yearlings.
^Privman, Jay (October 24, 2008). "Zenyatta completes perfect season". Daily Racing Form (Breeders' Cup). Retrieved September 7, 2011. "It was the perfect end to a perfect day by a perfect filly. Zenyatta, the best older female horse in the country, completed a perfect season. . . by winning the $2 million Ladies' Classic on Friday at Santa Anita's Oak Tree meeting, the first of two days of the 25th Breeders' Cup championships." (Zenyatta was foaled in 2004, making her a 4-year-old at the time.)