The funny thing is that a typical horse probably has closer to 2 or 3 horsepower and a big chonkin draft horse up to 4 or 5 times.

James Watt just picked the smallest possible value of horse when defining the unit so he could sell more steam engines.

https://www.youtube.com/watch?v=7qxTKtlvaVE (donut, How Much Horsepower is a Horse?)