While it is true that the maximum output of a horse is around 15 horsepower, when you average the output of a horse over the course of a work day it ends up being around a horsepower. Watt defined this amount as "the amount of work required from a horse to pull 150 pounds out of a hole that was 220 feet deep".
It's a stupid unit because it averages long-term output including resting periods, and was inveted for steam engine marketing. At least it reflects the quantity it measures, reducing confusion some people have with kW and kWh.
Imagine Marlboro man doing this... Then riding into the sunset just before he feels the need for a cigarette 🚬 and then boom! Horse parts everywhere mixed with guy parts. Meanwhile a car driving through the near street gets rained on with pieces of the good stuff... anus, balls, penis, and skin with bloody hair on it as well as eyeballs, fingers and other parts easily recognizable to the layman. That's why you never top off your horse!