Standardbreds are a breed of horse best known for their ability to race in harness at a trot or pace instead of under saddle at a gallop. Developed in North America, the breed is now recognized worldwide for its harness racing ability. They are solid, well-built horses with good dispositions that are also used under saddle for a variety of equestrian activities, particularly in the Midwest and eastern United States.

In the 17th century, the first trotting races were held in the Americas, usually in fields on horses under saddle. However, by the mid-18th century, trotting races were held on official courses, with the horses in harness. Breeds that have contributed foundation stock to the Standardbred breed included the Narragansett Pacer and the Canadian Pacer, Thoroughbreds, Norfolk Trotter, the Hackney, and the Morgan. Breeders selected bloodlines that would produce the fastest horses, with one of the most notable sires being the gray Thoroughbred stallion Messenger, who was exported to the United States in 1788. Messenger was bred to race on flat but, like many of the early Thoroughbreds, had Norfolk Roadster blood in his pedigree. He was mated to many Thoroughbred mares, and it was soon found that he had the ability to produce good trotting horses. Messenger's descendant, the legendary Hambletonian 10, also known as Rysdyk's Hambletonian, was born in 1849. He was sold, his owners thinking he was worthless, but later became one of the most prolific sires of Standardbreds, today with nearly every trotter or pacer tracing its lineage back to him. He had sired 1,335 offspring between 1851 and 1875. Another influential sire was the Thoroughbred "Diomed", born in 1777. When the sport started to gain popularity, more selective breeding to produce the faster harness trotter.