Although there are plenty of races for other breeds, the biggest races in the world are open only to Thoroughbred horses. Horses of this breed are known for their strength, speed and athleticism, and they are also used for other equestrian sports as well as racing.lthough there are plenty of races for other breeds, the biggest races in the world are open only to Thoroughbred horses. Horses of this breed are known for their strength, speed and athleticism, and they are also used for other equestrian sports as well as racing.