A foal is an equine up to one year old; this term is used mainly for horses, but can be used for donkeys. More specific terms are colt for a male foal and filly for a female foal, and are used until the horse is three or four. When the foal is nursing from its dam (mother), it may also be called a "suckling". After it 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 three (four in horse racing) is called a mare, and a colt over three is called a stallion. A castrated male horse is called a gelding regardless of age; however, colloquially, the term "gelding colt" is sometimes used until a young gelding is three or four. (There is no specific term for a spayed mare other than a "spayed mare".)

Horses that mature at a small stature are called ponies and occasionally confused with foals. However, body proportions are very different. An adult pony can be ridden and put to work, but a foal, regardless of stature, 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 stature, their body proportions are similar to that of an adult horse. Pony foals are proportionally smaller than adults, but like horse foals, they are slimmer and have proportionally longer legs.