The Marsh Tacky is a rare breed of horse found in the southern US, specifically in the swamps of South Carolina and Georgia. Like the Florida Cracker, Chickasaw, and a few other southern US breeds, the Marsh Tacky is of Spanish Colonial type. This means it is predominantly descended from the horses brought by Spanish settlers to the southeastern US.
Through the 1700s the southern US was culturally much closer to the Caribbean than to the English northeast. Most of the old southern breeds of horses, including the Mustang, are heavily influenced by the original Spanish stock, and their appearance and gaits show similarity to the Pasos of the Caribbean and Latin America. During the 1800s Thoroughbred & Arabian imports became a popular influence across the US, contributing to the more recent and still-popular breeds such as the Saddlebred and Morgan. The old Spanish type of horses survived in remote rural areas, where they were much-appreciated work and pleasure horses.
(Edited to add: my current research is revealing that this view of the Spanish influence may be oversimplified. I do not have any firm answers yet, but it is interesting to note that huge numbers of horses were bred in New England for export to the Caribbean during the 1700s. The Narragansett Pacer was one of the most famous breeds produced for this market, and must have had a significant impact. But more on that later.)
Having no particular use in the show ring, they have tended to be little known outside of their place of origin. Although still treasured by hunters and others who ride in the back country, where the Marsh Tacky's surefootedness, good sense, and courage is appreciated, the breed is so rare it is in danger of extinction: there are only about 100 Marsh Tackies left. The Carolina Marsh Tacky Outfitters have an excellent description of the horses and their history, and the site has beautiful photos of these rare and unique horses.
The American Livestock Breeds Conservancy is involved in studying the Marsh Tacky, and helping develop strategies for preserving it.
ETA: Here is another useful article about the Marsh Tacky.