found: Wikipedia, Apr. 20, 2007(A filly is a young female horse, too young to be called a mare. There are several specific definitions in use. A common developmental definition is a female animal that has not reached sexual maturity; this usually means it is under 24 months of age. However, horse owners more commonly define a filly as a female horse under the age of three years old. Horse-racing sets the cutoff age for fillies as younger than four years. The equivalent term for a male is a colt. When horses of either sex are very young, they can also be referred to as foals)
