That's not even remotely close to what the unit horsepower means at all.  It makes no mention of how high the horse lifts the load, and 746 is really the number for the conversion factor of HP to Watts.
What happened is, James Watt needed a performance metric to sell the steam engine, and compare it to something people already understood.  He studied the performance of draft horses, and how much load they could lift by pulling a mill wheel.  Based on the speed the horses walked, and the load the millwheel lifted, he determined that horses could pull with the power equivalent to lifting a 550 pound vertical load at a rate of 1 ft per second.