Filly
A filly is a young female horse typically under the age of four years, distinguished from a mare, which is a female horse four years or older.[1][2] The term originates from Old Norse "fylja," entering English in the 15th century, and its plural form is "fillies."[2] In equestrian terminology, all newborn horses are initially called foals, with females referred to as filly foals from birth; the term "filly" continues to apply to young female horses until they reach mare age.[3] Fillies are often trained for various disciplines, including racing, where they may begin competing as early as two years old, though racing eligibility can extend to under five years in some contexts.[4] Successful racing fillies hold significant value, as they can transition to broodmares for breeding after retiring from the track, potentially producing future champions and contributing to bloodline preservation in breeds like Thoroughbreds.[5][6] Beyond horses, "filly" informally refers to a spirited young woman or girl, a usage rooted in the animal's energetic connotation.[7] Culturally, fillies feature prominently in events like Australia's Champion Fillies Day, which celebrates female horse racing alongside fashion and social traditions.[2]Definition and Terminology
Definition
A filly is a female horse or other equine that is four years old or younger.[8] This classification encompasses horses from birth through their fourth calendar year, distinguishing them from adult females known as mares, which are five years old or older.[8] Equines are categorized by sex and age primarily for purposes of breeding, racing, and general management, allowing standardized handling and record-keeping across industries.[9] While the term filly technically includes newborn females, it often emphasizes those past the foal stage—typically under one year old and still nursing or recently weaned—up through the juvenile and early adolescent phases up to three years. Foals, by contrast, specifically refer to horses under one year of age, regardless of sex.[10] In standard equine age reckoning, particularly in the Northern Hemisphere, a horse's age advances on January 1 each year, regardless of its actual birthdate within the previous year; thus, a horse foaled in late 2024 is considered one year old starting January 1, 2025.[11] This convention facilitates uniform classification in competitive and regulatory contexts.[11]Related Terms
In equine terminology, a filly—a female horse under five years of age—is differentiated from related terms that account for sex, reproductive status, and maturity to ensure precise classification in breeding, racing, and general husbandry.[12] Key comparative terms include the colt, an intact male horse four years old or younger; the gelding, a castrated male horse of any age; the mare, a female horse five years old or older; and the stallion, an uncastrated adult male horse typically five years or older used for breeding.[12] These distinctions emphasize that fillies represent immature females, contrasting with adult females (mares) and various male categories based on castration and age.[13] The following table illustrates the intersections of equine sex, age, and status in a standard matrix, using Thoroughbred conventions where ages are calculated from January 1 of the birth year:| Age Category | Intact Male | Castrated Male | Female |
|---|---|---|---|
| Under 1 year (foal) | Foal colt | Foal gelding | Foal filly |
| 1–2 years (yearling) | Yearling colt | Yearling gelding | Yearling filly |
| 3–4 years | Colt | Gelding | Filly |
| 5+ years (adult) | Stallion/horse | Gelding | Mare |