Appendix Quarter Horse
A horse registered with the American Quarter Horse Association which is the result of breeding a Thoroughbred and an American Quarter Horse that has a permanent number, or a combination of an Appendix numbered American Quarter Horse and an American Quarter Horse with a permanent number. Appendix horses are distinguished by an "X" in front of their registration number and their certificates are gold.
Source:
American Quarter Horse Association http://www.aqha.com/