Here is a better history of the unit than I recall from other reading:
Savery used the term [horse-power] first when he devised a system
for rating his fire engines.His engine horsepower rating was determined on
the basis of the horses it displaced directly, plus those maintained in
pasture between the shifts necessary to keep an equivalent horse-operated
pump working 24 hours a day.
.... John Smeaton continued to use the term but fixed the work of one
horsepower as equivalent to the work a horse would do in lifting 22,916
pounds a height of one foot against the force of gravity.
Still other horsepower values appeared, and for many years the horsepower
unit had in fact little meaning.
Watt was determined to end this confusion.... He determined that the
average horse could raise one hundredweight [1cwt =112 lbs] to a height of
196 feet in one minute, and could continue working at this steady rate
until replaced by a relief horse. He then increased this figure by 50%, to
ensure that the purchasers of his engine would have no complaint, and
arrived at a figure that is equivalent to 33,000 ft-lbs per minute.