1 Answer

The following terminology is used to describe horses of various ages:

Foal: a horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at five to seven months of age, although foals can be weaned at four months with no adverse physical effects.   Yearling: a horse of either sex that is between one and two years old.   Colt: a male horse under the age of four. A common terminology error is to call any young horse a "colt",  actually only refers to young male horses.   Filly: a female horse under the age of four.Mare: a female horse four years old and older.   Stallion: a non-castrated male horse four years old and older.  Horse is sometimes used colloquially to refer specifically to a stallion.   Gelding: a castrated male horse of any age.

http://en.wikipedia.org/wiki/Horse