All modern Thoroughbreds have as common ancestors one or more of three stallions, the Byerly Turk, the Darley Arabian, and the Godolphin Barb, which were imported into Great Britain from the Middle East and North Africa between 1689 and 1724. Mated with strong English mares, they produced offspring with both speed and endurance. Thoroughbreds that compete in organized racing are registered in the official national stud books, or pedigree registers, of their country of birth. The British stud book was begun in 1791. Stud records in the United States date from 1873.

When horses destined for racing careers are two years old, they begin training that includes accepting a rider’s weight and commands. Although many two-year-olds race, Thoroughbreds are usually in their prime between the ages of three and five, and horses up to ten years of age have competed successfully. Some races are for horses of one sex only, but most races are open to entries of either sex.

 A female horse is known as a filly until its fifth birthday and as a mare thereafter. A castrated male horse of any age is called a gelding. A male horse that has not been castrated is known as a colt until its fifth birthday, when it is thereafter referred to simply as a horse or a stallion, regardless of its age.