The American Miniature Horse was bred to be the "ideal horse, in miniature". All critiquing of a full size horse can be applied to the Miniature Horse. The objective is to have a small, sound horse that is strong, agile and with a good disposition.
The miniature horse must be no taller than 34 inches for the A division, and no taller than 38 inches for the B division. The American Miniature Horse Association (AMHA) only recognizes miniature horses under 34". Only the American Miniature Horse Registry (AMHR) recognizes horses in the B division. There are many other registries around the world, but the vast majority of minis in the United States are registered under AMHA or AMHR (or both). Unlike larger horse breeds, the American Miniature Horse is NOT measured at the withers (shoulder). Instead, the horse is stood on a level surface with four feet square and perpendicular to the ground, and then the horse is measured at the last hair of the mane. There is much debate for this practice, but this is how it is done. A trick used by many is to trim the feet shorter or leave the heel longer to alter the horse's height. Also, because most minis are body clipped, they leave some of the body hair at the base of the mane as "fake hair" or if they want a taller horse they clip off some of the mane hair so it starts further up the shoulder.
Minis come in all colors, even some not seen in their larger counterparts. They come in all pinto genetics, all appaloosa genetics, all solid colors, they exhibit the dilute genes such as the creme gene, the silver gene, the greying gene, the dun gene, and the champagne gene. They have all eye colors. You will certainly find one in any color that's your favorite! They come in so many colors that oftentimes even color specialists can't figure out what they are!
Currently, as with any breed, there are "fads" which are followed in the show ring, according to which horses the judges place. I would have to say there is a big push for "Arabian Type" miniatures. Some miniature horses will closely resemble the breeds of larger horses, and because Arabs are valued for their beauty, many mini breeders are breeding for horses that exhibit similar "type" such as flagging their tails, flat toplines, hooky necks and ears, and a dished head. This does not mean that the American Miniature Horse MUST look like an Arab; it simply means that some people are breeding for those characteristics. Some minis look like little Draft horses, others like scaled-down Quarter Horses. This also does not mean that those breeds were BRED DOWN smaller; only that the minis happen to look like the larger breed either due to coincidence or a breeding program breeding for it.
There currently isn't a STRICT breed standard, in my opinion, other than the horse must be pleasing to the eye, move straight and balanced, and have excellent conformation in general. Remember that form follows function... or is it the other way around :-)
One of my big things to look for is hooves... many minis do not have the best feet. They either have boxy feet or club feet many times. Because minis are NOT allowed to be shod on show grounds, I have seen many trainers grow the foot unnaturally long for increased action in driving horses. I do NOT condone this! Natural action is BETTER for a breeding program AND the horse! The AMHR rulebook states for the breed standard: Hooves must be "round and compact, trimmed as short as practical for an unshod horse..." so PLEASE look after those feet!
One of the appealing things about the American Miniature Horse is that there is an emphasis (so far) on a NATURAL horse. Horses are shown as natural as possible with no glitter, braids, etc. allowed. The mane and tail must be full (no roached manes or docked tails) but they may be shortened and pulled.
When looking at a Miniature Horse to follow the breed standard... remember this is a breed based on HEIGHT ONLY. ANY horse, regardless of it's background, can be hardshipped in either AMHR or AMHA registries as long as they are small enough. And judges are to give preference to the SMALLEST possible horse... PROVIDED that the soundness and conformation are equal.
Dwarf miniature horses are part of the breed. In the past century, dwarf horses were purposely used in breeding programs because of their diminutive size (even compared to the Miniature!). Thus, the Dwarf gene is floating around in many bloodlines today, and it is difficult to locate as the gene has not been identified by geneticists. What is a Dwarf? A dwarf miniature horse is a horse that expresses the recessive gene. It is known that the dwarf gene is recessive... so a horse can CARRY the gene but not BE a dwarf. It is only when BOTH parents carry a gene and pass it on that they produce a true dwarf if they BOTH pass their gene. With the gene being recessive, it is almost impossible to tell if a horse carries the gene unless the horse is bred and throws a dwarf. The horse also must be bred to a horse that ALSO carries the gene. A horse could, theoretically, carry the gene and NEVER throw a dwarf... but it's children could carry the gene as well. Some breeders argue that ALL minis carry dwarf genetics, but this is scientifically highly improbable.
How do you know if a miniature is a dwarf? There are many "types" of dwarfs, some are more extreme than others. Usually dwarfism expresses itself in the following ways:
The horse is abnormally small, and does not follow a typical growth rate. They can be more long than tall. The horse's jaw is malformed with a bad bite. The legs are not straight, and often one or more legs are severely twisted. The horse's head is domed. They may suffer from varying health problems. They have large heads and almost no neck. They have pot bellies. Some dwarfs may only exhibit one of these characteristics, and in mild ways. Most registries refuse to register a dwarf, but some "Minimal Dwarfs" are offered papers anyway.
Dwarfs are often considered "cute" and "sweet", but it is not recommended that ANY responsible breeder breed for them purposely. There is a stigma against dwarfs, but they will occur in even the best of breeding programs. Dwarfs are extremely expensive in that they need excessive vet and farrier care above and beyond the average horse, and they do not live as long as the average miniature.