Miniature horses are less than three feet tall. Their small size makes them great service animals, and you can sometimes see them walking with their humans through neighborhoods, or even on buses. This is a big improvement from their old jobs. Until the 1950’s, miniature horses were used to work underground in coal mines since their small size allowed them to fit inside the tunnels. Miniature horses often live longer than larger horses, with an average lifespan of 25-35 years.