Until the early 1900's, horses powered plows and harvesting machines. Today, cars, trucks, and tractors have taken over those jobs.
The Spanish had royal horse farms running in Jamaica by 1515, some of which, Fransicco Pizarro took for his expididton to Peru.
Horses can't reproduce as fast as some of the other Big 5 farm animals. They can only have 1 foal per year.