Horsepower
As the Industrial Revolution produced mechanical devices to perform work, it was only natural to compare the productivity of an engine to that of the animals that had historically provided the muscle. So why don't we talk about oxenpower or bullpower? And why was horsepower actually based on the work capacity of Shetland ponies. And not to get too picky, why is "one horsepower" not the amount of work one horse could accomplish in a given time?