Its name is derived from its ability to outrun other horse breeds in races of 1⁄4 mi (0.40 km) or less; some have been clocked at speeds up to 44 mph (71 km/h).

The American Quarter Horse is also used in English disciplines, driving, show jumping, dressage, hunting, and many other equestrian activities.

[citation needed] As the Thoroughbred breed became established in America, many colonial Quarter Horses were included in the original American stud books.

[9] This began a long association between the Thoroughbred breed and what would later become officially known as the "Quarter Horse", named after the 1⁄4 mile (0.40 km) race distance at which it excelled.

On the Great Plains, settlers encountered horses that descended from the Spanish stock Hernán Cortés and other Conquistadors had introduced into the viceroyalty of New Spain, which became the Southwestern United States and Mexico.

[24] After winning the 1941 Fort Worth Exposition and Fat Stock Show grand champion stallion, the horse honored with the first registration number, P-1, was Wimpy,[25] a descendant of the King Ranch foundation sire Old Sorrel.

Other sires alive at the founding of the AQHA were given the earliest registration numbers Joe Reed P-3, Chief P-5, Oklahoma Star P-6, Cowboy P-12, and Waggoner's Rainy Day P-13.

[27] Other significant Thoroughbred sires seen in early AQHA pedigrees include Rocket Bar, Top Deck and Depth Charge.

[28] Since the American Quarter Horse was formally established as a breed, the AQHA stud book has remained open to additional Thoroughbred blood via a performance standard.

Horses listed in the appendix may be entered in competition, but offspring are not initially eligible for full AQHA registration.

Some breeders argue that the continued addition of Thoroughbred bloodlines are beginning to compromise the integrity of the breed standard.

[citation needed] With the internationalization of the discipline of reining and its acceptance as one of the official seven events of the World Equestrian Games, there is a growing international interest in Quarter Horses.

[37] The Quarter Horse has a small, short, refined head with a straight profile, and a strong, well-muscled body, featuring a broad chest and powerful, rounded hindquarters.

[citation needed] Horses shown in-hand in Halter competition are larger yet, with a very heavily muscled appearance, while retaining small heads with wide jowls and refined muzzles.

Not only are there concerns about the weight to frame ratio on the horse's skeletal system, but the massive build is also linked to hyperkalemic periodic paralysis (HYPP) in descendants of the stallion Impressive (see Genetic diseases below).

"[41] The show hunter type is slimmer, even more closely resembling a Thoroughbred, usually reflecting a higher percentage of appendix breeding.