It took about 300 years for specialists and enthusiasts to breed Thoroughbreds, and this process is still under way. They have a very rich Arabian ancestry, and have contributed their genes to many famous breeds of horses. The Thoroughbred stands 16 hands high, and its body is muscled and gracious. Thoroughbreds occur in bay, dark bay, chestnut, black or gray. White markings on the muzzle and legs are permissible.