What term is used to describe a horse that is less than a year old?

The term used to describe a horse that is less than a year old is "foal." This designation applies specifically to the very young horse, regardless of its gender, from birth until it reaches one year of age. Foals are generally characterized by their youthful appearance and behavior, and they are often seen nursing from their mothers.

While "colt" refers to a young male horse and "filly" to a young female horse, both of these terms apply to animals that may be older than a foal, specifically in the case of colts, until they are about four, and fillies, until they are about three or four, depending on the breed. The term "yearling" is used for horses that are one year old but not yet two. Thus, "foal" is the correct term for horses younger than a year, encompassing both sexes.