James Watt invented the term horsepower. He measured the work a horse would do in a 10 hour day, took the average. Keep in mind in James Watt's improved the steam engine and wanted an idea how many horses his engine could replace. Horses were used to lift coal out of a mine. So James Watt came up with the term 550 ft-lb of work in a second. This unit can also be converted in to circular units too. Let's say you have a pulley with a rope lifting a 55 lb weight. If the outer speed of the pulley is 10 ft/s, then you are using one horsepower, 55 lb x 10 ft/sec = 550 ft-lb/sec. Things can get a little more complicated if you want to talk about torque. Let's say you have 550 lb-ft of torque turning at 1 rev per second, you will have 550 x 1 x 2 pi = 3454 ft-lb/sec = 6.28 hp
Keep in mind I put my Physic book away a long time ago. Sure someone may think this isn't right. When computing hp and using circular unit, revolutions must be converted to radians per second. 1 revolution is 2 pi radians.
Google James Watt and see what the experts have to say about horsepower and torque.
As for a horse, yes it can pull a 20 hp lawn mower backwards. Bet a horse can't pull a 20 hp IH C backwards. Difference, old tractors measured true horsepower at the wheels, PTO, or belt.
Didn't briggs get in trouble for over inflating the hp ratings?