- In most cases, a 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.
Fillies are sexually mature by age two and are sometimes bred at that age, but generally should not be bred until they themselves have stopped growing, usually by age four or five. Some fillies may exhibit estrus as yearlings.
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.
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- Hammond, Gerald (2000). The Language of Horse Racing. London: Taylor & Francis. p. 79. ISBN 1-57958-276-1. OCLC 44923115.
- 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.)
- Ensminger, M. E. Horses and Horsemanship: Animal Agriculture Series. Sixth Edition. Interstate Publishers, 1990. ISBN 0-8134-2883-1 p. 149-150
- James R Gillespie (2000). Modern Livestock and Poultry Production. Thomson Delmar Learning. ISBN 978-0-7668-1607-7.