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Horsepower is a measure it took a horse to go around one revolution of some sort of farm wheel. That terminology was standardized and obviously passed down.
HP = RPM x T / 5252 (this constant is derived from the 33,000 / pi x 2) The 33,000 is ft.lbs per minute that the force the horse would create around that wheel.
It is strange, because we could have easily had donkeypower, or elephant power if you really think about it.