From Wikipedia... I learn something every day..

[color:"blue"]The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute [/color]