The two most famous breeds that originated in the US are the American Quarter Horse and the Morgan Horse.

The Quarter Horse was first bred early in the 17th century in Virginia and other settlements on the East Coast. This oldest of Amerian breeds was derived from the native horses introduced by long ago Spanish explorers and English Thoroughbreds imported to Virginia in 1611. From this wonderful mixture of hardiness and strength blossomed the American Quarter Horse - a compact chunky horse averaging 15 hands high with incredibly powerful hindquarters. Quarter horses have always been working horses - in the early days of our country they worked on the farm, rounded cattle, hauled goods to and from the market and later drew pioneer wagons across the plains. They were versatile, for unlike the draft breeds they could easily be ridden. However the Quarter Horse rapidly became renowned for its incredible sprinting speed and English settlers lost no time in developing races, often between plantations or even on village streets. These quarter mile races were an important part of early colonial social life. As Thoroughbreds were introduced to the Eastern United States, distance racing became more popular and more and more oval race tracks were constructed. The Quarter Horse moved west where its legendary ability to spin on a dime and its uncanny "cow-sense" made him a cowboy's favorite companion. He continues to excel in ranch and rodeo work, as well as trail riding, English and Western pleasure and performance classes, the hunter-jumper arena and gymkhana. In addition, quarter horse racing is again becoming popular, especially in the Western US.
There are 11 foundation Quarter Horse families, but the most notable sires are Sir Archy and Janus. Characteristics of the Quarter Horse include a small muzzle; a neat, short and wide head; big, broad, flat knees: along underline with a well sprung barrel; short cannons with low-set hocks for maximum flexion and heavy muscular hindquarters.