Then I found this, which is more my understanding (other than the age statement at the end):
Many horse experts will tell you that any full grown equine that measures under 14.2 hands at the withers is classified as a pony. This isnít always the case, however. A pony is typically considered an equine under 58 inches tall when it reaches adult height, but a pony also has other distinctive characteristics other than just its small size. For example, most pony breeds have thick, broad bodies and thick necks. They also have legs that are proportionately shorter for their body than a horse exhibits. Some pony breeds also have broad heads, especially through the forehead, along with larger eyes. Some ponies also have large hooves for their size.
On the other hand, miniature horses are even smaller than most pony breeds. Most registries wonít allow membership to a mini thatís taller than 34 inches when full grown. There are miniature horses, however, as tall as 38 inches. A miniature horse is also built differently than a pony. Ideally, a mini is a scaled-down version of a horse, with a slimmer build than a pony, and longer legs for its size. The head is also in proportion to the body, as are the feet. Also, a miniature horse does not have the heavy bone often associated with pony breeds. In essence, a miniature horse is usually more refined than a typical pony. A mini is longer lived than most ponies, too. They have an average life span of 25-35 years.
Funny since minis ARE bred down Shetlands....it's just a size thing...under 34" AMHA..34"-38" AMHR...over 38" Shetland registry...
I have a 35" shetland that is registered as AMHR...Their all the same breed, just sizes differentiate which registry their in.