We need 550 ft/lbs per second for 1
hp.
So we put a rope on our horse, run it over a pulley-on-a-tree, to a 550 lb. wt. on the ground. If the horse can pull it up 10 feet in 10 seconds he's a 1 horse-power horse( actually a little more, since there is some friction loss in the system. To make it a little more manageable "gear-wise" we can use 275 lbs., in which case he'll have to pull it 20 feet in 10 seconds, or 10 feet in 5 seconds. Any weight can be used, if the math is adjusted to suit,... a race horse might be able to put out impressive
hp with a light weight, same as a small high-revving engine. but for heavy loads, the high-torque/low-geared draft-horse would be the way to go. HTH :-)
Larry