The American Quarter Horse, or Quarter Horse, is an American breed of horse that excels at sprinting short distances. Its name is derived from its ability to outrun other horse breeds in races of a quarter mile or less; some have been clocked at speeds up to 44 mph (70.8 km/h). The development of the Quarter Horse traces to the 1600s.

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The American Quarter Horse is the most popular breed in the United States today, and the American Quarter Horse Association is the largest breed registry in the world, with almost three million living American Quarter Horses registered in 2014. The American Quarter Horse is well known both as a race horse and for its performance in rodeos, horse shows and as a working ranch horse.

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The compact body of the American Quarter Horse is well suited for the intricate and quick maneuvers required in reining, cutting, working cow horse, barrel racing, calf roping, and other western riding events, especially those involving live cattle. The American Quarter Horse is also used in English disciplines, driving, show jumping, dressage, hunting, and many other equestrian activities.

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Early foundation sires of Quarter horse type included Steel Dust, foaled 1843; Shiloh (or Old Shiloh), foaled 1844; Old Cold Deck (1862); Lock's Rondo, one of many "Rondo" horses, foaled in 1880; Old Billy—again, one of many "Billy" horses—foaled circa 1880; Traveler, a stallion of unknown breeding, known to have been in Texas by 1889; and Peter McCue, foaled 1895, registered as a Thoroughbred but of disputed pedigree. Another early foundation sire for the breed was Copperbottom, foaled in 1828, who tracks his lineage through the Byerley Turk, a foundation sire of the Thoroughbred horse breed.

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The main duty of the ranch horse in the American West was working cattle. Even after the invention of the automobile, horses were still irreplaceable for handling livestock on the range. Thus, major Texas cattle ranches, such as the King Ranch, the 6666 (Four Sixes) Ranch, and the Waggoner Ranch played a significant role in the development of the modern Quarter Horse. The skills required by cowboys and their horses became the foundation of the rodeo, a contest which began with informal competition between cowboys and expanded to become a major competitive event throughout the west. To this day, the Quarter Horse dominates in events that require speed as well as the ability to handle cattle.

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Sprint races were also popular weekend entertainment and racing became a source of economic gain for breeders. As a result, more Thoroughbred blood was added to the developing American Quarter Horse breed. The American Quarter Horse also benefitted from the addition of Arabian, Morgan, and even Standardbred bloodlines.

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In 1940, the American Quarter Horse Association (AQHA) was formed by a group of horsemen and ranchers from the Southwestern United States dedicated to preserving the pedigrees of their ranch horses. 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,[23] 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.[24] The Thoroughbred racehorse Three Bars, alive in the early years of the AQHA, is recognized by the American Quarter Horse Hall of Fame as one of the significant foundation sires for the Quarter Horse breed.[25] Other significant Thoroughbred sires seen in early AQHA pedigrees include Rocket Bar, Top Deck and Depth Charge