Here's why:
* Yearling: This term refers to a horse that is between one and two years old. This age range encompasses the period when a horse is transitioning from a foal to a young adult.
* Colt: A male horse under four years old.
* Filly: A female horse under four years old.
While a half-grown wild horse can be called a colt or filly, depending on its sex, yearling is a more general and accurate term.