The Spanish Mustang Registry, founded in 1957, describes the breed as follows:

The true Spanish Mustang is a direct descendant of the horses brought to the New World by the early Spaniards. Confused by many with the feral horses currently managed by the Bureau of Land Management (BLM), there is a vast difference in both appearance and ancestry.

The Spanish Mustang is a medium sized horse ranging from 13.1 to 15 hands with an average size of approximately 14.2 hands with proportional weight. They are smooth muscled with short backs, rounded rumps and low set tails. Coupling is smooth and the overall appearance is of a well balanced, smoothly built horse. The girth is deep, with well laid back shoulder. They possess the classic Spanish type head with a straight or concave forehead and a convex nose which is in contrast to the straight forehead and nose of most breeds. Ears are average to short and usually notched or curved towards each other. Necks are fairly well crested in mares and geldings and heavily crested in mature stallions. Chests are narrow but deep with the front legs joining the chest in an "A" shape rather than straight across. Chestnuts are small or missing altogether, particularly on the rear legs. Cannons are short, upper foreleg is long with the canon bone having a larger circumference than other breeds of comparable size and weight.