A filly horse is a female horse that is too young to be called a mare. There are two specific definitions in use: in most cases, a filly is a female horse under four years old. In some nations, such as the United Kingdom and the United States, the word filly may be used to describe a female horse under five years old. When horses of either sex are less than one year, they are referred to as foals. Horses of either sex between one and two years old may be called yearlings. The equivalent term for a male horse is a colt.