The Appaloosa is best known for its distinctive, leopard complex-spotted coat, which is preferred in the breed. Spotting occurs in several overlay patterns on one of several recognized base coat colors. There are three other distinctive, "core" characteristics: mottled skin, striped hooves, and eyes with a white sclera.

Skin mottling is usually seen around the muzzle, eyes, anus, and genitalia. Striped hooves are a common trait, quite noticeable on Appaloosas, but not unique to the breed. The sclera is the part of the eye surrounding the iris; although all horses show white around the eye if the eye is rolled back, to have a readily visible white sclera with the eye in a normal position is a distinctive characteristic seen more often in Appaloosas than in other breeds. Because the occasional individual is born with little or no visible spotting pattern, the ApHC allows "regular" registration of horses with mottled skin plus at least one of the other core characteristics. Horses with two ApHC parents but no "identifiable Appaloosa characteristics" are registered as "non-characteristic," a limited special registration status.

There is a wide range of body types in the Appaloosa, in part because the leopard complex characteristics are its primary identifying factors, and also because several different horse breeds influenced its development. The weight range varies from 950 to 1,250 pounds (430 to 570 kg), and heights from 14 to 16 hands (56 to 64 inches, 142 to 163 cm). However, the ApHC does not allow pony or draft breeding.

The original "old time" or "old type" Appaloosa was a tall, narrow-bodied, rangy horse. The body style reflected a mix that started with the traditional Spanish horses already common on the plains of America before 1700. Then, 18th-century European bloodlines were added, particularly those of the "pied" horses popular in that period and shipped en masse to the Americas once the color had become unfashionable in Europe. These horses were similar to a tall, slim Thoroughbred-Andalusian type of horse popular in Bourbon-era Spain. The original Appaloosa tended to have a convex facial profile that resembled that of the warmblood-Jennet crosses first developed in the 16th century during the reign of Charles V.

The old-type Appaloosa was later modified by the addition of draft horse blood after the 1877 defeat of the Nez Perce, when U.S. Government policy forced the Native Americans to become farmers and provided them with draft horse mares to breed to existing stallions. The original Appaloosas frequently had a sparse mane and tail, but that was not a primary characteristic, as many early Appaloosas did have full mane…

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Recent research has suggested that Eurasian prehistoric cave paintings depicting leopard-spotted horses may have accurately reflected a phenotype of ancient wild horse. Domesticated horses with leopard complex spotting patterns have been depicted in art dating as far back as Ancient Greece, Ancient Persia, and the Han Dynasty in China; later depictions appeared in 11th-century France and 12th-century England. French paintings from the 16th and 17th centuries show horses with spotted coats being used as riding horses, and other records indicate they were also used as coach horses at the court of Louis XIV of France. In mid-18th-century Europe, there was a great demand for horses with the leopard complex spotting pattern among the nobility and royalty. These horses were used in the schools of horsemanship, for parade use, and other forms of display. Modern horse breeds in Europe today that have leopard complex spotting include the Knabstrupper and the Pinzgau, or Noriker horse.

The Spanish probably obtained spotted horses through trade with southern Austria and Hungary, where the color pattern was known to exist. The Conquistadors and Spanish settlers then brought some vividly marked horses to the Americas when they first arrived in the early 16th century. One horse with snowflake patterning was listed with the 16 horses brought to Mexico by Cortez, and additional spotted horses were mentioned by Spanish writers by 1604. Others arrived in the western hemisphere when spotted horses went out of style in late 18th-century Europe, and were shipped to Mexico, California and Oregon.
The Nez Perce people lived in what today is eastern Washington, Oregon, and north central Idaho, where they engaged in agriculture as well as horse breeding. The Nez Perce first obtained horses from the Shoshone around 1730. They took advantage of the fact that they lived in excellent horse-breeding country, relatively safe from the raids of other tribes, and developed strict breeding selection practices for their animals, establishing breeding herds by 1750. They were one of the few tribes that actively used the practice of gelding inferior male horses and trading away poorer stock to remove unsuitable animals from the gene pool, and thus were notable as horse breeders by the early 19th century.

Early Nez Perce horses were considered to be of high quality. Meriwether Lewis of the Lewis and Clark Expedition wrote in his February 15, 1806, journal entry: "Their horses appear to be of an excellent race; they are lofty, eligantly [sic] formed, active and durable: in short many of them look like fine English coarsers [sic] and would make a figure in any country." Lewis did note spotting patterns, saying, "... some of these horses are pided [pied] with large spots of white irregularly scattered an…

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Located in Moscow, Idaho, the ApHC is the principal body for the promotion and preservation of the Appaloosa breed and is an international organization. Affiliate Appaloosa organizations exist in many South American and European countries, as well as South Africa, Australia, New Zealand, Canada, Mexico and Israel. The Appaloosa Horse Club has 33,000 members as of 2010, circulation of the Appaloosa Journal, which is included with most types of membership, was at 32,000 in 2008. The American Appaloosa Association was founded in 1983 by members opposed to the registration of plain-colored horses, as a result of the color rule controversy. Based in Missouri, it has a membership of more than 2,000 as of 2008. Other "Appaloosa" registries have been founded for horses with leopard complex genetics that are not affiliated with the ApHC. These registries tend to have different foundation bloodstock and histories than the North American Appaloosa. The ApHC is by far the largest Appaloosa horse registry, and it hosts one of the world's largest breed shows.

The Appaloosa is "a breed defined by ApHC bloodline requirements and preferred characteristics, including coat pattern". In other words, the Appaloosa is a distinct breed from limited bloodlines with distinct physical traits and a desired color, referred to as a "color preference". Appaloosas are not strictly a "color breed". All ApHC-registered Appaloosas must be the offspring of two registered Appaloosa parents or a registered Appaloosa and a horse from an approved breed registry, which includes Arabian horses, Quarter Horses, and Thoroughbreds. In all cases, one parent must always be a regular registered Appaloosa. The only exception to the bloodline requirements is in the case of Appaloosa-colored geldings or spayed mares with unknown pedigrees; owners may apply for "hardship registration" for these non-breeding horses. The ApHC does not accept horses with draft, pony, Pinto, or Paint breeding, and requires mature Appaloosas to stand, unshod, at least 14 hands (56 inches, 142 cm). If a horse has excessive white markings not associated with the Appaloosa pattern (such as those characteristic of a pinto) it cannot be registered unless it is verified through DNA testing that both parents have ApHC registration.

Certain other characteristics are used to determine if a horse receives "regular" registration: striped hooves, white sclera visible when the eye is in a normal position, and mottled (spotted) skin around the eyes, lips, and genitalia. As the Appaloosa is one of the few horse breeds to exhibit skin mottling, this characteristic "...is a very basic and decisive indication of an Appaloosa." Appaloosas born with visible coat pattern, or mottled skin and at least one other characteristic, are registered with "regular" papers and have full show and b…

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