The truth, though, is that ponies and horses are not two stages of development of a single animal. They're actually different, although related and quite similar, creatures. Ponies remain small when they're fully grown.
All types of horses are known as equines. Equine experts measure the height of animals in units of measurement called hands. A hand is approximately four inches, which is an average width of a human hand.
The main distinction between ponies and horses is height. A horse is usually considered to be an equine that's at least 14.2 hands (or about four feet ten inches) tall. A pony, on the other hand (pun totally intended!), is an equine less than 14.2 hands.
This is not a black and white rule, though. There are horses less than 14.2 hands tall, and there are ponies that are taller than 14.2 hands. Why? The answer is that ponies and horses are different in other important ways beyond just their height.
Pony breeds, in addition to being shorter than horses, have other characteristics that make them different from horse breeds. Ponies and horses have different conformations. This means that they have differences in their bone structures, muscles and overall body proportions.
For example, ponies tend to be stocky and stronger (for their size) than horses. They are more tolerant of cold weather and have good endurance, which makes them good work horses. Ponies also tend to be very intelligent. From a human point of view, this means that ponies might be more stubborn than a horse.
Physically, ponies usually have thicker manes and coats that help protect them from the cold. They have shorter legs and wider chests, as well as heavier bones, thicker necks and shorter heads.
Likewise, there are horse breeds that don't grow taller than 14.2 hands, yet don't share the same characteristics of ponies. Examples of these breeds include Caspian, American Miniature, Morgan and Icelandic horses. Despite their shorter stature, these equines are considered horses because they are quite different from ponies.