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. 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.

Source: Wikipedia