Watt, a Scottish inventor, was trying to quantify the power of his steam engine in the late 18th century. He observed that a strong horse could lift a weight of 33,000 pounds one foot in one minute. He standardized this as the unit of power, which we know today as one horsepower.
This was a clever way to relate the power of his invention to something people could easily understand. However, it's important to note that the actual power output of a horse varies greatly, and Watt's "horsepower" was an idealized value.