From your car, to your lawn mower, to your snow blower, to your chainsaw—the power of almost every engine you deal with is measured in terms of horsepower. None of these things seemingly have anything to do with horses, so where did that measurement come from?
Well, actual horses. In the early days of the steam engine, inventor and engineer James Watt made some significant improvements to the Newcomen steam engine, and attempted to market and sell his new version. This was easy enough when Watt was dealing with people who already had a Newcomen engine, and could directly compare the output of the old engine to his models. Some potential customers didn’t have a steam engine at all yet, though, and were still using draft horses to move weight and power machinery. He needed a way to explain his engine in terms that these people would understand.
He worked out the idea of horsepower while watching some draft horses lift coal at a mine (in some versions of the story, he watched a horse turn a mill wheel). He determined that a horse could do 33,000 foot-pounds of work in a minute (for example, move 330 pounds of coal 100 feet in a minute, or haul 33 pounds of coal 1000 feet in the same time), and dubbed that “one horsepower.” He then used that as a way to make the apples-to-oranges comparison between horses and his machinery, and could say that a steam engine had power equivalent to X number of horses.