"A filly is a young female horse 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."