The American Quarter Horse is an American bree of horse that excels at sprinting short distances. Its name came from its ability to outdistance other breeds of horse in races of a quarter mile or less, where some individuals have been clocked at speeds up to 55 mph. 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 over 3 million American Quarter Horses registered worldwide.