Horsepower as a measure of power wasn't invented until the 1700s when James Watt, a Scottish inventor and engineer, came up with the idea. He probably picked horses because they were commonly used back then for everything from riding to pulling wagons to carrying burdens.
Power is work done over time. And Watt made 1 Horsepower equal to 550 foot-pounds per second. All this means is that he thought a horse could lift 550 pounds a distance of 1 foot in 1 second. Kind of an odd measurement, but hey, it was the 1700s and Watt was busy redesigning the steam engine and inventing the paper copier in his spare time. Yes, he made a paper copier in the 1780s, long before the office cubicle was invented.
Michio Kaku's book PHYSICS OF THE FUTURE mentions in one section that throughout most of our history, people have had to make do with about 1/5 horsepower, or in other words, our own strength to accomplish tasks. This got me thinking about humans and our ingenuity.
Now any hardcore gearhead or mechanic can tell you the Horsepower of their car. For example, the 2012 Honda Civic has 140 Horsepower. A quick glance at history may give you the idea that until steam engines came along, humans were stuck with their weak 1/5 Horsepower. And taking into account the smaller size of ancient people, I'd say we were working with more like 1/10 Horsepower.
But humans have been domesticating animals for a long time, giving us access to the strength of horses, oxen, llamas, donkeys, goats, elephants, dogs and others. An ancient Egyptian warrior in a chariot would have commanded 2 or 4 Horsepower. The average ox weighs about twice as much as a horse, so a farmer with a team of 4 oxen would have 8 Horsepower.
Elephants were used in north Africa and Asia, especially India and Thailand. The average Asian elephant would have cranked out about 9 Horsepower, and the bigger African elephant would be pushing 12 Horsepower. Pretty impressive, and scary if the elephant is chasing you.
Going beyond domesticated animals, humans did pretty well for themselves by using mechanical advantages. In ancient Greece, the Athenians built boats with 3 rows of oars called triremes. The standard trireme used in the battle of Salamis in 480 BC had 174 rowers, or about 17 Horsepower. Enough to move the trireme to ramming speed when calm days rendered its sails useless.
Throughout human history, we've been harnessing Horsepower through animals or mechanical advantage. So today when you start your car or your lawn mower, think about the amazing Horsepower at your command.