Prior to the age of four, female horses are called fillies, and from age four and up, they are called mares. Female horses can also be called yearlings when they are between one and two years old, or foals before they are a year old.
Male horses can also be called foals and yearlings. Young male horses that have not been castrated are called colts until they turn four, when they earn the name of stallion. Male horses that have been castrated are called geldings at any age. The term "rig" is often used to refer to a male horse that was not properly castrated or has other testicular problems.