The Appendix Quarter Horse is a subtype of the Appendix Horse. This subtype accepts only solid-colored crosses between an American Quarter Horse and a Thoroughbred or an Appendix Quarter Horse.

Origins:
Crossing Quarter Horses with Thoroughbreds was popular at the beginning of the 20th century, when breeders tried to improve the stamina of their Quarter Horses.

In 1949, the American Quarter Horse Association (AQHA) added an appendix to its studbook, in which all of the crossbred horses were registered. These horses, who were designated with numbers that begin with an 'X,' were allowed to compete in AQHA events and therefore received the name "Appendix Quarter Horses." Today, crossing American Quarter Horses with Thoroughbreds is still popular, and one can find many Appendix Quarter Horses.

Breeding and advancement into the main studbook:
In order to be registered as an Appendix Quarter Horse, a horse must have one Quarter Horse parent, and one Thoroughbred or Appendix Quarter Horse parent.

According to the rules of the AQHA, Appendix Quarter Horses which are registered in the AQHA's appendix are eligible for advancement into the main American Quarter Horse studbook. In other words, these Appendix Quarter Horses can be upgraded into a Quarter Horse status. However, this can happen only if the Appendix horse has an excellent conformation, an outstanding performance, and no genetic defects. In Horse Isle, though, this form of advancement is not available.

Performance metrics:
The following are the: range, average, (SD), and MOE of performance metrics of ordered Appendix Quarter Horses in Horse Isle (not bred ones). In rare cases, horses might have metrics outside of the range. Breeders can produce horses that are beyond this range.

Speed: 15.9-18.4, 17.3 (0.4), 0.09.
Sprint: 67-80, 75 (3), 0.64.

Accel: 1.06-1.24, 1.15 (0.04), 0.01.
Decel: 0.97-1.16, 1.08 (0.04), 0.01.

Jump: 5.16-5.46, 5.30 (0.07), 0.01.
Pull: 2.28-3.20, 2.75 (0.19), 0.04.

Turning: 58.43-72.39, 66.27 (3.22), 0.63.
Reverse: 2.7-3.4, 3.0 (0.1), 0.03.

Stamina: 48.68-55.70, 52.41 (1.53), 0.3.
Reaction: 0.82-0.90, 0.86 (0.02), 0.00.

Coats & Height:
Appendix Quarter Horses can come in all of the possible coat colors except for dominant black and extensive mealy (minimal-mealy does exist). The coat is always solid.

Height: 14.2hh to 17hh.