A foal is an equine, particularly a horse, that is one year old or younger. More specific terms are colt for a male foal and filly for a female foal, but these terms are used until the horse is age three or four. When the foal is nursing from its dam (mother), it may also be called a suckling. After the young horse has been weaned from its dam, it may be called a weanling. When a mare is pregnant, she is said to be "in foal." When the mare gives birth, she is "foaling" and the impending birth is usually stated as "to foal." A newborn horse is "foaled".

After a horse is one year old, it is no longer a foal, and is a yearling. There are no special age-related terms for young horses older than yearlings. When young horses reach breeding maturity, the terms change: a filly over the age of three (four in horse racing) is called a mare and a colt over the age of three is called a stallion. A castrated male horse is called a gelding, regardless of age, though colloquially the term "gelding colt" is sometimes used until a young gelding is three or four years old. (There is no specific term for a spayed female horse, they are simply "spayed mares".)

Horses that mature at a small size are called ponies and are occasionally confused with foals. However, body proportions are very different. An adult pony can be ridden and put to work, while a foal, regardless of size, is too young to be ridden or used as a working animal. Foals, whether they grow up to be horse or pony-sized, can be distinguished from adult horses by their extremely long legs and small, slim bodies. Their heads and eyes also exhibit juvenile characteristics. Although ponies exhibit some neoteny with the wide foreheads and small size, their body proportions are similar to that of an adult horse. Pony foals are proportionally smaller than adults, but like horse foals, are slimmer and proportionally longer-legged than their adult parents.

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