What’s the Difference Between a Filly, Colt, and Mare?

Last updated: July 12, 2026

By: Miles HenryFact Checked

Filly, colt, foal — these terms get mixed up constantly, but they mean different things.

Horse age and sex terms — complete reference

Term	Sex	Age	Notes
Foal	Either	Under 1 year	General term for a horse under 1 year old.
Weanling	Either	Typically 4–7 months to under 1 year	Foal that has been weaned from its mother.
Yearling	Either	1–2 years old	No longer a foal.
Colt	Male	Under 4 years old	Intact male horse.
Filly	Female	Under 4 years old	Young female horse.
Gelding	Male	Any age	Castrated male horse.
Stallion	Male	Usually 5 years and older	Intact adult male, especially in breeding use.
Horse	Male	5 years and older	Racing-form term for an intact male age 5 and older.
Mare	Female	5 years and older	Adult female horse.

Filly and Colt: Definitions and Differences

A filly is a female horse four years old and younger. A colt is an intact male horse four years old and younger. Both terms are age- and sex-specific — they don’t apply to all young horses, and they stop applying once the horse turns five. At five, a filly becomes a mare and a colt becomes a stallion (if he remains intact) or stays a gelding (if he’s been castrated).

The physical differences between colts and fillies become more pronounced as they age. Colts tend to develop more muscular builds and can show more assertive behavior as testosterone increases. Fillies generally mature slightly earlier and are often described as more focused in training — though individual temperament varies widely within each sex and breed. Neither sex is inherently better or worse for any discipline; it depends on the individual horse.

Foal, Weanling, and Yearling

These three terms describe the same horse at successive stages of its first two years. A foal is any horse under one year old, male or female. Once that foal is weaned from its mother — typically between four and seven months — it becomes a weanling while still under a year old. In racing terminology, the weanling becomes a yearling on January 1 following its birth year, a term that applies until the horse turns two.

Weaning is a significant milestone. It marks the point where the foal transitions from relying on the mare for nutrition to eating grass, hay, and feed. The timing matters — wean too early and the foal misses critical nutrition and social bonding; wean too late and the mare’s condition can suffer. Most breeders aim for five to six months as the standard weaning window.

Yearlings are in early training in Thoroughbred racing — this is when they’re broken to saddle, beginning the conditioning process that leads to their racing debut as two-year-olds. For more on that timeline, see: when do Thoroughbred horses start racing.

Mare, Stallion, and Gelding

These three terms describe adult horses and are essential for anyone reading a racing form or evaluating a horse for purchase. A mare is a female horse five years old or older. A stallion is an intact male horse five years old or older used for breeding — on racing forms, this horse is listed as “horse,” not stallion. In breeding circles, the term is stallion; on racing forms the official designation is horse. A gelding is a castrated male horse — the castration removes the designation of age, so a gelding is always a gelding regardless of whether he’s three or fifteen. Official Thoroughbred age rules are maintained by the Jockey Club.

Geldings are the most common type of horse in general use. Castration is typically performed between six months and two years and reduces hormone-driven behavior that can make intact males difficult to manage in training and competition. Most male racehorses that don’t enter breeding programs are gelded. Some of the greatest racehorses ever were geldings — Kelso, John Henry, and Forego among them.

How These Terms Work on a Racing Form

Racing forms list every horse by color, sex, and age. Once you know the terminology, the shorthand becomes readable quickly. A male horse between two and four years old is listed as a colt unless he’s been castrated — then he’s a gelding, which stays on his entry for life. At five, an intact male becomes a horse on the form. On racing forms, the designation “horse” specifically means an intact male age five or older — not stallion, not colt, simply “horse.” Fillies are listed as fillies through age four, then as mares starting at five. Racing form terminology follows conventions used by the Daily Racing Form.

Horse Age and Sex Terms in Spanish

If you work in a barn with Spanish-speaking horsemen — common at Louisiana tracks — these are the terms you’ll hear alongside the English equivalents.

Horse age and sex terms — English and Spanish

English term	Spanish term	Notes
Foal	Potro / Potra	Potro = male foal; Potra = female foal
Filly	Potranca	Young female horse — comparable to filly
Colt	Potro	Young intact male horse
Mare	Yegua	Adult female; potranca y yegua are not the same
Stallion	Semental	Intact adult male used for breeding
Gelding	Caballo castrado	Castrated male
Yearling	Añojo	One-year-old horse

¿Potranca y yegua es lo mismo? No. Potranca is a young female horse (comparable to filly), while yegua is an adult mare — the same distinction as filly vs. mare in English.

Many races are restricted by sex — fillies and mares run against each other in conditions races, allowance races, and stakes events. It’s not unusual for a filly or mare to compete against males and succeed. Some of the greatest racehorses were females — Ruffian, Winx, and Zenyatta among them.

FAQs About Horse Age and Sex Terms (Filly, Colt, Foal)