The Thoroughbred is a horse breed best known for its use in horse racing. Although the word thoroughbred is sometimes used to refer to any breed of purebred horse, it technically refers only to the Thoroughbred breed. Thoroughbreds are considered  hot-blooded  horses, known for their agility, speed and spirit.The Thoroughbred as it is known today was developed in 17th-and 18th-century England, when native mares were crossbred with imported Oriental stallions of Arabian, Barb, and Turkoman breeding.horoughbreds in the United States have historically been used not only for racing but also to improve other breeds. The early import Messenger was the foundation of the Standardbred, and Thoroughbred blood was also instrumental in the development of the American Quarter Horse.