A young horse is referred to as a foal. If it is a male, it can also be called a colt, while a female is known as a filly. During the period when the foal is nursing, it is sometimes called a suckling. Once it is no longer nursing and has been weaned, it is then called a weanling.

After the horse reaches the age of one year, it is no longer considered a foal but becomes a yearling.

Even though the horse is no longer referred to as a foal, a young adult horse may still be called a colt or a filly. As a colt matures past a certain age (which varies depending on the breed), he is commonly referred to as a horse. Similarly, when a filly reaches that particular age, she is then referred to as a mare. A male horse that has been castrated is known as a gelding, regardless of his age.

A stallion is an intact male horse that has been used for breeding purposes, regardless of his age. On the other hand, any female horse that has been bred is referred to as a mare, regardless of her age (at least in the case of Thoroughbreds).