The Puerto Rican Criollo is a gaited breed of mixed ancestry* (that's what "criollo" means) with a variety of gaits. The slowest gait is a short-stepping fast walk, called the paso fino (which can be seen in this video). The paso largo is a lengthened, faster running walk. Like the speed rackers of the United States and the Icelandic Horse, some Puerto Rican Criollos can do a singlefoot gait (called andadura) at tremendous speeds. Some may pace at high speeds. Racers, called Caballos de Andadura, are often raced drag-race style in short sprint heats, two-by-two.
Though in videos one sometimes seems to hear and see pacers, still photos (see this page) show that most of them are actually singlefooting (only one foot is touching the ground; a pacer has the two feet on the same side touching the ground simultaneously). Here is a video of a horse where one can clearly hear the rapid four-beat footfalls at top speed.
Despite the high speeds it appears to be traditional to race the horses bareback. Like the Icelandic Horse these Criollos are small in stature but clearly powerful and quite capable of carrying large adults with little effort. If you read Spanish, there is a little more information in forums such as this one.
Interesting to note: there is a website here (in Spanish) about the Andadura Gallega, or Galician Singlefooter, in Spain. Check out their photos of race days. Again, it is not clear this is a specific breed, as on the stallion page they are standing what appears to be an American Standardbred horse.
*The PR Criollo is likely descended to some extent from the Spanish jennet and Barb, but also likely to have northern blood, such as from the Narrangansett Pacer, which was exported in great quantities to the Caribbean. The silver dapple color seen in some Puerto Rican Criollos is, to my knowledge, distinctive to northern European bloodlines. Read more about this complex heritage in this post about the Rocky Mountain Horse.