A yearling is a young horse, older than a foal but not yet two years old. Yearlings are generally too young to ride.
While the word yearling usually refers to a filly or a colt — a horse between the ages of one and two, of either sex — you can use it to talk about any animal of that age. It might sound a little funny, but go ahead and describe your one year old cat as a yearling. The word combines year, from the Old English gear, and the diminutive suffix -ling, which is often used in words describing young animals, like gosling and duckling.