Fun fact, a typical horse exerts about 1 horse power of usable work while performing. That's so weird, I'm sure almost no one would've been able to guess that - but it's true.
(To be clear, that's sustained effort over time, not just momentary. Athletically trained humans can do about 1 HP of peak momentary effort, and around 0.3 HP if sustained over time.)
And a horse can do quite a bit more in peak as well— 1 HP is definitely meant to be the long term continuous output of a typical horse under load, especially a consistent load such as turning a millstone.
Track cyclists (sprinters, world class) do 2KW+ peak for a few seconds at a time. That's potentially ~3HP. (and while doing so, average more than 70kph over a 200m distance)
