Horsepower is averaged based on an extended time period, how much power a horse can put out on average while constantly working. They can't do 15 hp on a constant basis, they can only do it for a relatively brief period of time, so their average is 1 hp. A 15 hp engine can run at 15 hp for a much longer period of time, which a horse can't do. If the engine was hypothetically capable of working consistently without ever breaking down, it would be able to run at 15 hp indefinitely. But even with the machine's lifespan in mind, it can still run for years at the same output, which is impossible for a horse.