A foal is a horse of either sex in its first year of life, from birth to the end of that year. A male foal is a colt foal, a female a filly foal. Foals nurse for the first months, begin sampling forage within weeks, and are usually weaned around four to six months of age.
See also: the male foal by sex, the dam who nurses the foal
Further reading: Foal on Wikipedia; management of reproduction in horses from the Merck Veterinary Manual.