how horse power=746watt?

Answer 1

...and to further muddle things, the METRIC hp was determined to be 75 m-kg, about 1.0139 greater than the English hp.
(Those metric horses must be a little stronger!)

Answer 2

It was a term invented by James Watt to sell his steam engine a while ago and it was basically how much work a horse could do over time. When his mechanical definition was converted to Watts, it came out to be that strange number....

Her's more than you want to know about it --

http://en.wikipedia.org/wiki/Horsepower#History_of_the_term_.22horsepower.22