The two checks, done the quiet way
A young Mustang is sexed by anatomy like any other foal: one look under the tail, one feel along the belly. Under a filly's tail there are two openings, the anus and below it the vulva, a short slit tapering at its lower end. Under a colt's tail there is only the anus. Along a colt's belly, midway underneath, sits the sheath, a clear fold of skin a flat hand can find; a filly's belly runs smooth back to her small udder. The checks themselves take seconds. With a Mustang foal — especially a wild-born or recently gathered one — everything around the checks takes longer, because a foal who does not trust people will not stand for any of it until the handling feels safe.
Why testicles are the last thing to rely on
New Mustang owners often hunt for testicles and stop there, which misleads in two directions. A colt's testicles may be descended and obvious, may appear over his first weeks, or may rest up in the body where they cannot be felt at all — so feeling nothing does not make a foal a filly. Equally, a filly's small udder and teats between her hind legs can be mistaken in a quick grope by someone expecting to find something. Rest the decision on the two dependable structures: vulva under the tail means a filly, sheath on the belly means a colt. If you happen to watch the foal urinate undisturbed, a stream from the belly supports colt and a stream from under the tail supports filly — a useful cross-check, never the main event.
Size, toughness, and temperament tell you nothing
Mustangs invite guessing because they arrive with reputations: the big, bold foal gets called a colt, the slight, watchful one a filly. In reality fillies grow just as tough and colts just as wary, and nutrition and birth timing move a foal's size far more than its sex does. Colour and markings have no link to it either. Let the anatomy answer, because the sex you settle on drives real decisions for a Mustang: gelding plans, which pen a youngster can live in, and what the adoption or title paperwork should say.
Handling a foal who has not chosen to trust you
Do not chase, corner hard, or separate a young Mustang from its mare to get this look — a frightened first handling colours every handling after it, and a panicked foal can hurt itself on fencing. Work in a small pen with the mare present, move at the foal's pace, and build up over short sessions: standing near, a hand resting on the body, then the tail lift and belly touch folded into ordinary grooming. Many owners find the moment comes naturally while scratching a foal who has started seeking contact. If the foal came through an adoption programme, the paperwork may already record a sex — treat that as a claim to verify with your own calm check, not as gospel, because foals sexed quickly in a busy facility are occasionally misrecorded.
Confirmation, and why accuracy matters early
If a wild-born foal will not yet allow a clear look, wait rather than forcing one; the structures grow easier to read, and the vet can confirm the sex at the foal's first examination, which a recently adopted Mustang needs anyway. Accuracy matters because an unrecognised colt left running with fillies becomes a breeding problem sooner than most owners expect, and a mis-sexed record follows the horse through every transfer. When the answer is certain, add it to the foal's record in KinsLe that day — out on a windy yard, Pet Pass is how you show a ranger, vet, or adopter exactly which Mustang this is.