When most of us start out learning about horses, we are told that a horse is a member of Equus caballus that is over 14 hands high (a hand, we quickly learned, is 10 cm). A pony, on the other hand, was defined as a member of the same species that was less than 14 hands. Now, miniature horses are always much smaller than 14 hands, so why are they called miniature horses rather than ponies?

The answer lies in the proportions. Most ponies are cobby in build (rounded and stocky - think of drawings by the cartoonist Thelwell), while many breeders of miniature horses (often affectionately referred to as "minis") prefer to breed for proportions that are more like that of a "regular" horse. The American Miniature Horse Registry (AMHR) suggests that a "mini" is ideally "a small, sound well-balanced horse" that, if photographed without anything to give a sense of size, should look identical to a full-sized horse.

As expected, size matters. The AMHR recognizes two categories of miniature horses: those in the "A" division are less than 82 cm (34 inches) at the withers, which is defined as being the last hair of the mane, while the "B" category covers horses 34–38 inches at the withers. The American Miniature Horse Association, however, does not recognize horses over 34 inches. A miniature horse is always measured in centimeters or inches, but never in hands.
If they were measured in hands, the largest "minis" would be around 8 or 9 hands (by way of comparison, an average Shetland pony measures 10 hands). "Minis" come in all color types, including the more exotic types like pinto and palomino as well as the more mundane bays, greys and chestnuts. Whatever size of color they are, miniature horses have been in the world for a surprisingly long time.

In the Renaissance and Restoration periods, they were occasionally found in private menageries as curiosities. At the other end of the economic spectrum, miniature horses also had their uses as pit ponies hauling coal in the mines.
There are also some who hold the view that some of the fossilized horse remains classified as ancestors of Equus caballus are actually the fossilized remains of "minis." There may be some uncertainty about the dating, but the fossils certainly have the right size and skeleton structure.

Since the 20th century, miniature horses have been developed as a breed and refined to the animals that they are today, and this work is ongoing. The most widely known breed of miniature horse is the Falabella, which originated in Argentina.

"Minis" are still kept as curiosities in petting zoos and farm parks, but they also have a wider range of roles. They are, of course, unsuited as riding animals, because of their size, but are often shown competitively and can be used for light harness work. Their most noteworthy work is therapeutic, as they are often used to provide interaction and comfort to the elderly, and in working with autistic children.