It's not metric, it is an old German unit. An old French unit is the cheval vapeur, and another one is the poncelet, which is about 981 W
1 pferdestarke=735.499W=0.98632hp=approx 1 cheval vapeur
They are all units of power and hence can be measured in many ways. There are DIN and SAE standards for measuring engine power, in particular.
Cheers
Greg Locock