The name came about thanks to James Watt who built early steam engines. He needed a way to sell them to people currently using horses as their beasts of burden. Comparing the power of a steam engine to the power of a horse made a lot of marketing sense. It’s not just marketing though, because there’s a whole bunch of math behind horsepower. Here come the numbers.
Horsepower is defined as 33,000 foot-pounds per minute. This is the amount of power needed to lift 330 pounds in one minute. Watt estimated that this was roughly the capabilities of the average horse. Not all horses are created equal, so there’s obviously room for error if you tried this out with a bunch of real horses. It’s what Watt came up with studying horses at a local coal mine, so your mining horses could perform differently.
Regardless, the definition of 33,000 foot-pounds per minute stands. Big horsepower numbers in a car mean it can move further in less time and that means a faster car. For comparison, the average human can manage 1.2 horsepower briefly, trained athletes can manage 2.5 horsepower, and a real horse can hit 14.9 horsepower for a few seconds. You’d need a herd of horses hooked up to your wagon to get the horsepower packed into today’s cars.