An engine is most often rated ate its peak power so although a car may have a 120 HP engine it is not necessarily always using or needing produce that power level but yes given 1 hp is 746 watts that does make it a roughly 90 KW power source.
The problem of harnessing creature power is that living things tend to not want to put out their full physical working capacity for any longer than necessary which means with living things laziness/conservation of energy always wins.
Crazy!?! I had a Mouse once, bloomin thing was up all night on his wheel! plus don't rats eat garbage? its like the most efficient and friendly incinerator ever!
Boy you have big mice! I bet he wouldn't run all night if you hooked a little dc motor to his wheel and made him charge a battery. Probably wouldn't weigh half a pound for long after all that exercise either.