Horse-racing sport. In harness racing, Standardbred horses are harnessed to lightweight, two-wheeled, bodiless (seat-only) vehicles known as sulkies. The sport's origins date to ancient chariot races. Today two types of horses are used, trotters and pacers. The former employ a gait in which the legs move in diagonal pairs, the latter a gait in which the legs move in lateral pairs. Since the establishment of pari-mutuel racing under lights in the 1940s, the sport has grown tremendously in popularity.