Thoroughbreds 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.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.