The competitive racing of horses is one of humankind's most ancient sports, originating among the prehistoric nomadic tribesmen of Central Asia who first domesticated the horse about 4500 BC. By the time humans
began to keep written records, horse racing was an organized sport in all major civilizations from Central Asia to the Mediterranean. Both chariot and human-mounted horse racing were events in the ancient Greek Olympics by 638 BC, and the sport became a public obsession in the Roman Empire. The origins of modern racing lie in the 12th century, when English knights returned from the Crusades with swift Arabian horses. Over the next 400 years, an increasing number of Arabian stallions were imported and bred to English mares to produce horses that combined speed and endurance. Matching the fastest of these animals in two-horse races for a private wager became a popular diversion of the nobility. Soon, horse racing became known as
“The Sport of Kings.”
The only horses that can be called "Thoroughbreds" are those descended from horses registered in the General Stud Book, published in 1791 by James Weatherby, whose family served as accountants to the members of the Jockey Club, and who traced the pedigree (the complete family lineage) of every horse racing in England. From 1793 to the present, members of the Weatherby family have continuously and meticulously recorded in subsequent volumes of the General Stud Book the pedigree of every foal born to those race horses. The American Stud Book was started in 1868. Thoroughbreds are so inbred that the pedigree of every single Thoroughbred horse can be traced back to one of only three stallions, called the "foundation sires." These stallions were the Byerley Turk, foaled c.1679; the Darley Arabian, foaled c.1700; and the Godolphin Arabian, foaled c.1724.