A "filly" is a young female horse, typically under the age of four. This term is used to distinguish young females from colts, which are young males. Fillies are known for their playful and energetic behavior as they grow and develop. As they mature, fillies can be trained for various purposes, including riding, racing, or work. Once they reach the age of four, they are usually referred to as mares. The development of a filly is an important stage in the life of a horse, influencing its future abilities and characteristics in the equestrian world.