The word "draft" represents not a specific kind of animal, but (according to Webster's Unabridged Dictionary) any animal used for pulling heavy loads. For some reason, Americans have come to use the word "draft" in reference to heavy horses, but a horse of any size may be trained to pull a load.

The heavy breeds are best for heavy farm work and/or large acreage. They weigh 1,600 pounds or better and stand at least 16 hands high from ground to withers (one hand equals 4 inches). About 95 percent of all heavy horses in America are either Belgian (originating in Belgium) or Percheron (originally from France).

Heavy horses found in North America are: American Cream, Belgian, Clydesdale, Percheron, Shire, Suffolk and Spotted. Heavy breeds that are common in Europe, but not found in North America, include: Ardennais, Comtois, and the Rhinelander.
For smaller acreage or lighter work, such as skidding (dragging) logs for firewood or fence posts, a sturdy saddle-type horse will do just fine. Like any work horse, it must be heavy-boned, well-muscled, and properly conditioned. Light horses suitable for draft work include the  Morgan and Quarter horse. Draft Ponies which are suitable for limited acreage or hilly terrain, one might consider the Heflinger or a Norwegian Fiord.

Equestrian International maintains inventories of draft horse bits suitable for work or competition.