A colt is a male horse under the age of four. A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses. A filly is a female horse under the age of four. A foal is a horse of either sex less than one year old. A yearling is a horse of either sex that is between one and two years old. A mare is a female horse four years old and older. A stallion is a non-castrated male horse four years old and older. A gelding is a castrated male horse of any age...like at that point it really cares. Who comes up with this stuff?