When a registered Quarter Horse is crossed with a registered Thoroughbred horse, the resulting foal is considered an Appendix Quarter Horse. american appendix horse association, inc. email@example.com 888-607-2242 Known for their docile disposition and great speed over short distances, these horses have been very popular in the horse racing industry in the United States. This relates to first generation crosses, not later breedings between two appendix animals. Created when the American Quarter Racing and the National Quarter Horse Breeders associations merged in 1949, the Appendix registry was intended to be a catchall for horses formerly registered with those groups. An appendix horse is a cross between a registered quarter horse and a registered thoroughbred. What is an appendix quarter horse? That is true but, even purebred Quarter Horses can be mostly thoroughbred. The Appendix Quarter Horse is a breed of American horses that developed from the Thoroughbred bloodlines. The ideal result is a horse with the quarter horse's calmer disposition and the thoroughbred's athletic ability. This is due in part to the Appendix section of the Quarter Horse registry.