A male horse used for breeding is called a stallion, whereas a neutered male horse is called a gelding. The term foal refers to a male or female horse that is less than a year old.
A male horse that is less than 5 years old also is called a colt, while a female under age 5 is a filly. A stallion of more than 5 years old is simply referred to as a horse. A stallion that impregnates a female horse is called the sire of the foal. Yearlings are male and female horses that are in their second year of life.