I just came to this realization that ponies aren’t baby horses.
People who are unfamiliar with horses may confuse an adult pony with a young, immature horse. While foals that will grow up to be horse-sized may be no taller than some ponies in their first months of life, their body proportions are very different. A pony can be ridden and put to work, while a foal is too young to be ridden or used as a working animal. Foals, whether they grow up to be horse or pony-sized, can be distinguished from adult horses by their extremely long legs and slim bodies. Their heads and eyes also exhibit juvenile characteristics. Furthermore, in most cases, nursing foals will be in very close proximity to a mare who is the mother (dam) of the foal. While ponies exhibit some neoteny with the wide foreheads and small size, their body proportions are similar to that of an adult horse.
You can read more on ponies in Wikipedia.