Filly/ˈfɪli/ Definition 1 of 2Pro Tip 1/3Age Specific Refers to female horses that are typically under four years old. The two-year-old filly has just started training. Pro Tip 2/3Equine Terminology Used in horse-related contexts, especially in racing and breeding. That filly comes from a line of champions. Pro Tip 3/3Potential for Confusion Don't confuse a 'filly' with a 'colt', which is a young male horse. The farmer separated the colts from the fillies.