What do you call a female horse that is under the age of four?

A female horse under the age of four is referred to as a filly. This term is specifically used to distinguish young female horses from both mares, which are adult females typically four years of age and older, and colts, which denote young male horses. Understanding these terms is important in horse evaluation and breeding contexts, as they provide insight into the horse's age and developmental stage, which can affect traits such as size, behavior, and suitability for certain roles in riding and competition. The term pony usually refers to a small horse, not specifically related to age or gender, making it not relevant in this case.