In practice, however...
- Horespower is a more or less arbitrary definition of power. Most horses would be able to deliver a short term power in excess of 1 hp. Might work out for the long term power, though, as explained by BOSFLASH. Heck, in the short term, a trained human athlete can exceed 1 hp.
- which definition of horsepower do you want to use - Germany, UK, US all have different definitions
- which horse(s) do you want to use? Different breeds have different power and ways to use them. Haflinger's may not exactly what you'd use to win a horse race, but if you could handle a team of 200 Haflingers, they'd simply drag your car across the field - sideways if necessary. The car simply would have no chance to get its power onto the ground, you'd actually be comparing the friction of 800 hooves (and ~100 tons of horse muscle) against 4 wheels. On the other hand, those beasts would be hard pressed to go faster than 20 kph.
-