A yearling is a young horse that is between one and two years old). Yearlings are comparable in development to a very early adolescent and are not fully mature physically. While they may be in the earliest stages of sexual maturity, they are considered too young to be breeding stock). Yearlings may be further defined by sex, using the term "colt" to describe any male horse under age four, and filly for any female under four). Generally, the training of yearlings consists of basic gentling on the ground; most are too young to be ridden or driven. Yearlings are often full of energy and quite unpredictable. Even though they are not fully mature, they are heavier and stronger than a human and require knowledgeable handling). Some breeding farms tend to leave yearlings alone to grow in pastures and natural settings, while others keep them stabled and condition them intensively for show or sale).

In addition to horses, the term "yearling" can also be used to describe any animal that is one year old or in the second year of its age. For example, a yearling lamb is one that is born between September 1 and December 31 of the year preceding the exhibit year.

It is worth noting that hunters are notorious for referring to fawns as "yearlings".