Trouble
Trouble is technically considered a miniature horse.  Even though any equine animal smaller than 12 hands is considered a pony.  Trouble is so small that he is only about 7 hands which makes him less than 3 feet tall. Being this small, he is in a category known as a Miniature Horse.

History of Miniature Horses
In the 17th century, miniature horses were bred as pets for Europe's Habsburg nobility. Records from the court of the French King Louis XIV, circa 1650, note the presence of tiny horses among the exotic creatures in the king's zoo. Paintings and articles featured the miniature horse by 1765. In England, Lady Estella Hope and her sisters carried on a breeding program from original English lines into the mid-nineteen hundreds. However, not all early miniatures were pampered pets of kings and queens. Some were used to work in the English Midlands, Wales and Northern European coal mines as pit ponies.

Assistance animals 
A miniature horse working as a service animal. There is controversy over whether miniature horses are suitable as assistance animals for the blind. Those who favor their use point out that horses live much longer than dogs and can be trained to perform similar tasks. Another plus is that some individuals, particularly from Muslim cultures, consider dogs unclean, but accept horses. Opponents of their use raise concerns that miniature horses are prey animals, with a fight or flight instinct that may limit their usefulness, and for legal reasons. In the US, where they are legally classified as livestock and require outdoor stabling for good health, their use is limited to owners with access to a large yard in communities having tolerant land use regulations. In terms of practical considerations, they note that it is difficult for even a miniature horse to do things such as lie down in the seat of a taxicab or to stay in a hotel room for extended periods of time.