The origins of term horsepower has nothing to do with Horses, but with Ponies!
Ok, so technically a Pony is part of the horse family, but most people tend to think of them as different enough that the statement still stands. It might surprise you to know that the term horsepower came from ponies and not horses. Here's how it happened:
An engineer called James Watt coined the term. Watt is famous from improving the performance of steam engines and for being talked about whenever we talk about how many watts a lightbulb has. He was working with ponies lifting coal at a mine, and he wanted to have a way to talk about the power available to these animals.
He calculated that, on average, a pony could do 22,000 foot-pounds of work in a minute. Then, he arbitrarily increased that amount by 50% to 33,000 foot-pounds per minute and called that one horsepower.