The Thoroughbred, as it is known today, was developed in 17th- and 18th-century England, when native mares were crossbred with imported stallions of Arabian, Barb, and Turkoman breeding.

All modern Thoroughbreds can trace their pedigrees to three stallions originally imported into England in the 17th and 18th centuries, and to a larger number of foundation mares of mostly English breeding.

Thoroughbreds are used mainly for racing, but are also bred for other riding disciplines such as show jumping, combined training, dressage, polo, and fox hunting.

Thoroughbred racehorses perform with maximum exertion, which has resulted in high accident rates and health problems such as bleeding from the lungs.

There are several theories for the reasons behind the prevalence of accidents and health problems in the Thoroughbred breed, and research on the subject is ongoing.

[3][5] Thoroughbreds are classified among the "hot-blooded" breeds, which are animals bred for agility and speed and are generally considered spirited and bold.

It was then that handicapping, a system of adding weight to attempt to equalize a horse's chances of winning as well as improved training procedures, began to be used.

[21][22][notes 1] Another was the Brownlow Turk, who, among other attributes, is thought to be largely responsible for the gray coat color in Thoroughbreds.

[12] According to Peter Willett, about 50% of the foundation stallions appear to have been of Arabian bloodlines, with the remainder being evenly divided between Turkoman and Barb breeding.

Further, as a percentage of contributions to current Thoroughbred bloodlines, Curwen's Bay Barb (4.2%) appears more often than the Byerley Turk (3.3%).

[33] Selective breeding for speed and racing ability led to improvements in the size of horses and winning times by the middle of the 19th century.

[37] Soon after the start of the 20th century, fears that the English races would be overrun with American-bred Thoroughbreds because of the closing of US racetracks in the early 1910s, led to the Jersey Act of 1913.

The act was not repealed until 1949, after which a horse was only required to show that all its ancestors to the ninth generation were registered in a recognized Stud Book.

Diomed, who won the Derby Stakes in 1780, had a significant impact on American Thoroughbred breeding, mainly through his son Sir Archy.

Diomed, who won the Derby Stakes in 1780, had a significant impact on American Thoroughbred breeding, mainly through his son Sir Archy.

[48] Lexington went on to a career as a breeding stallion, and led the sires list of number of winners for sixteen years, fourteen of them in a row.

The early import Messenger was the foundation of the Standardbred,[53] and Thoroughbred blood was also instrumental in the development of the American Quarter Horse.

Oriental horses were imported into South Africa from the late 17th century in order to improve the local stock through crossbreeding.

[73] The number of Thoroughbred foals registered each year in North America varies greatly, chiefly linked to the success of the auction market which in turn depends on the state of the economy.

[80] The Thoroughbred industry is a large agribusiness, generating around $34 billion in revenue annually in the United States and providing about 470,000 jobs through a network of farms, training centers, and race tracks.

Thus the practice prevents an oversupply of Thoroughbreds, although modern management still allows a stallion to live cover more mares in a season than was once thought possible.

[95] Record prices at auction often grab headlines, though they do not necessarily reflect the animal's future success; in the case of The Green Monkey, injuries limited him to only three career starts before being retired to stud in 2008, and he never won a race.

The best-known example of this was the 1986 Kentucky Derby winner Ferdinand, exported to Japan to stand at stud, but was ultimately slaughtered in 2002, presumably for pet food.

[113] In addition to racing, Thoroughbreds compete in eventing, show jumping and dressage at the highest levels of international competition, including the Olympics.

[122] Crossbred horses developed from Thoroughbreds, (informally categorized as "hot bloods" because of temperament) crossed on sturdy draft horse breeds, (classified as "cold bloods" for their more phlegmatic temperament) are known as "warmbloods," which today are commonly seen in competitive events such as show jumping and dressage.

[129] Thoroughbreds also have other health concerns, including a majority of animals who are prone to bleeding from the lungs (exercise induced pulmonary hemorrhage), 10% with low fertility, and 5% with abnormally small hearts.

If proper remodeling occurs before hard training and racing begins, the horse will have a stronger musculoskeletal system and will have a decreased chance of injury.

[129] Studies have shown that track surfaces,[136] horseshoes with toe grabs,[130] use of certain legal medications,[137] and high-intensity racing schedules may also contribute to a high injury rate.

[128] The level of treatment given to injured Thoroughbreds is often more intensive than for horses of lesser financial value[139] but also controversial, due in part to the significant challenges in treating broken bones and other major leg injuries.

Previously hopeless cases can now be treated,[140] and earlier detection through advanced imaging techniques like scintigraphy can keep at-risk horses off the track.