An Appendix Horse is a horse who is a cross between a Thoroughbred or an Appendix Quarter Horse with an American Quarter Horse or an American Paint Horse or an Appendix Quarter Horse.

Origins of the 'X' Quarter Horse:
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".

The Appendix Quarter Horse today:
In order to be registered in the AQHA's appendix, which still exists today, a horse must have one Quarter Horse parent, and one Thoroughbred or Appendix Quarter Horse parent. Today, crossing American Quarter Horses with Thoroughbreds is still popular, and one can find many Appendix Quarter Horses.

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, a solid coat color, and no genetic defects. In Horse Isle, though, this form of advancement is not available.

The Appendix Horse:
In 2003, a new and independent registry for Appendix Quarter Horses was established, with the aim of allowing breeders to breed pinto Appendix horses, or to breed their Appendix horses to Appendix or Thoroughbred horses.

This registry, which is not related to the AQHA, accepts Quarter X Thoroughbred crosses, as well as American Paint Horses X Thoroughbred crosses. It also allows crossings between Appendix horses and Quarter, Thoroughbred, and Paint horses, which means that the Appendix horse can have more than 50%, or less than 50%, Thoroughbred blood.

In Horse Isle, the generic 'Appendix Horse' allows crosses with Paint horses, and, therefore, horses in this group can have a pinto coat. The 'Appendix Quarter Horse' is a subtype of the 'Appendix Horse' breed. Breeding of Appendix Quarter Horses in Horse Isle must be according to the rules of the AQHA, which were listed above.

Performance metrics:
The following are the: range, average, (SD), and MOE of performance metrics of ordered Appendix 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: 16.4-18.3, 17.3 (0.4), 0.08.
Sprint: 68-83, 74 (3), 0.65.

Accel: 1.05-1.26, 1.15 (0.05), 0.01.
Decel: 1.01-1.16, 1.08 (0.04), 0.01.

Jump: 5.15-5.45, 5.30 (0.07), 0.01.
Pull: 2.22-3.22, 2.73 (0.18), 0.04.

Turning: 59.81-74.42, 66.39 (2.85), 0.56.
Reverse: 2.7-3.4, 3.0 (0.1), 0.03.

Stamina: 48.19-55.28, 52.04 (1.44), 0.28.
Reaction: 0.82-0.89, 0.86 (0.02), 0.00.

Coats & Height:
Appendix Horses can come in all of the possible coat colors and patterns in the game except for dominant black, leopard, and extensive mealy (minimal-mealy does exist).

Height: 14.2hh to 17hh.