The term Thoroughbred describes a breed of horse whose ancestry traces back to three foundation sires -- the Darley Arabian, the Godolphin Arabian and the Byerly Turk.
Named after their respective owners -- Thomas Darley, Lord Godolphin and Captain Robert Byerly -- these three stallions were brought to England from the Mediterranean Middle East around the turn of the 17th century and bred to the stronger, but less precocious, native horse.
The result was an animal which could carry weight with sustained speed over extended distances, qualities which brought a new dimension to the burgeoning, aristocratically-supported, sport of horse racing.

Physical Appearance
The average height of today's Thoroughbred is a little over 16 hands, as opposed to the 14-hand average height of the horses from which the breed originated.
If there is one place where these attributes are reflected, it is the shape and carriage of the head and the look of the eye. The head should be correctly proportioned to the rest of the body, displaying a good flat forehead and wide-set intelligent eyes. Carried relatively low, the head should sit well on a neck which is somewhat longer and lighter than in other breeds.
The withers should be high and well-defined, leading to an evenly curved back. The shoulder should be deep, well-muscled and sloped along the same parallel as that on which the head is carried. From the point of the shoulder, the forearm should show adequate muscling which tapers towards a clean-looking knee which in turn tapers into the full width of the cannon. This in turn should be short and comparatively flat, with the tendons distinctly set out and clean.
The pastern should be neither too long nor too short and set at an angle a little less than 45 degrees to the vertical. When viewed from behind or in front, the legs should be straight and move smoothly in unison through one plane.
Power comes from the hindquarters and all-important is that the bone structure of the upper hind leg is such as to make room for long, strong muscling. These driving muscles act between the hip bone and the thigh bone which should be long and the angle it makes with the hip bone wide.