One is bred down, the other is built stocky
A miniature horse is a horse in every proportion, simply scaled down: the legs are slim and in proportion to the body, the head is refined, the neck arches like a show horse's, and the whole animal looks like a full-sized horse photographed from far away. Registries set strict height ceilings for minis — measured at the last hairs of the mane, they stand far below pony height — and breeders select hard to keep the proportions horse-like as the size shrinks. A pony is a different shape of animal entirely: short legs under a deep, round barrel, a thick neck and coat, and a sturdy, chunky build at any height. Put a mini beside a pony of similar height and the difference is obvious at a glance.
How each came to be
Miniature horses were developed deliberately by breeders crossing small individuals over generations, often with Shetland and other small breeds in the background, selecting always for the horse look in miniature. Ponies, by contrast, are the product of place: native breeds shaped by centuries of poor grazing and hard weather in mountains, moorland, and islands. That history explains their builds — the pony's stockiness is survival equipment, while the mini's elegance is a breeding goal. It also explains why pony breeds are named for their homelands while minis are registered by size and type.
What each can do — and the mini's limit
Ponies are working animals: they are ridden by children and small adults, driven, and shown, and a fit pony is a genuine athlete. Miniature horses are driven in light carts, shown in hand over jumps and obstacle courses they tackle without a rider, used as therapy and visiting animals, and kept as companions — including as companions for full-sized horses. The firm limit is riding: a mini's back and legs are not built to carry weight, and putting even a small child on one risks injuring the animal. This is the misunderstanding that matters most, because a mini looks, to an uninformed buyer, like the perfect first mount for a toddler. It is not a mount at all.
Care: more alike than different
Both eat like the thrifty small equines they are: small quantities of plain forage, restricted rich grazing, and vigilance against obesity and laminitis. Minis with finer coats may need rugs and shelter in weather that hardy native ponies winter through unrugged. Both need the same farriery cycle, dentistry, and parasite control as any horse, and both are herd animals that need equine company — a mini is frequently the company, bought precisely to keep a big horse settled. Dental problems from crowded teeth in a small skull are a known watch-point in minis as they age.
Buying with your eyes open
Choose a pony if you want an animal that can be ridden or worked; choose a mini if you want a companion, a driving prospect within its limits, or a small equine to handle from the ground. The price, insurance, and showing worlds of the two barely overlap, so buy the description, not just the height. Sellers carry a duty to describe a mini honestly as unrideable, however cute the saddle photographs online may look. When a miniature horse changes hands through KinsLe, its Transferrable Profiles record describes it accurately and carries its care history with it, so no new owner mistakes it for a child's pony.