One horsepower is defined as the amount of power required to lift 550 pounds one foot in one second.
The maximum output of a horse is around 15 horsepower
The average deadlift for a male 21-year-old is 2.6 times bodyweight. Depending on the weight class, deadlifts will range from 344lb to 582lb
It seems likely to me that there was once a man nicknamed 'horse' for his above average strength. He was used as the unit of measurement of power. Unfortunately for him, his feat has been lost to history.