Funnily enough, horses actually produce much more than 1 horsepower. I think the story goes that James Watt estimated it would take a team of 10 horses resting and taking turns to pull a tractor/mill/whatever that keep 2 horses constantly busy, so that's how he calculated it.

But over shorter durations, horses can produce much more.

https://en.wikipedia.org/wiki/Tiananmen_Square