Is horsepower ACTUALLY equivalent to the number of horses that the number says it is?

No. It was supposed to be an approximation based on comparing draft horses to steam engines.

It’s a unit of measurement, like pounds, kilograms, watts, or meters. Although there’s quite a few different uses of the term so it means different things in different contexts, none of them really have anything to do with horses anymore. (Good info to have).

Kind of like carats used in gemstone measurements no longer refer to the seeds they were originally named for. The name remains but the unit has been standardized.

Even with cars there’s some big differences when people reference horsepower in different areas, mostly having to do with how they get the measurement and what’s included. (at the flywheel, at the wheels, with accessories, dyno variations, etc.)

It’s related to how much a horse could lift in a given period of time.

Since the “horsepower” is a standard unit equal to 33,000 feet-pounds per minute, and horses, while arguably mass-produced, are not standardized, the two cannot be functionally nor semantically equivalent.

In other words, some horses have more horsepower than others.

Of course, the way horsepower is measured complicates matters. There are several different ways of measuring a car’s horsepower, and the rated or advertised horsepower is often an estimate rather than an average of measured data. Two otherwise identical cars with “300 horsepower” might have very different amounts of power if, for example, one has 300 “shaft horsepower” and one has 300 “wheel horsepower”.

tl;dr: No.

No, just like one foot is not the length of every human foot in existence. There will be some variation, but it is about right.

As @koanhead implies, 1 Horsepower doesn’t always equal 1 Horsepower. Depending on who you go by, 1HP = 735 to 745.7 Watts.

Also, even engines of the same HP behave differently due to their power delivery. Done right, even a 1 HP engine could pull as hard as 300 horses… though it would do so verrrrrry sloooooowwwwwwllllllyyyy. I bring this up since HP is not a reliable measure of what an engine can do. For instance, which do you think is quicker on the drag strip; a 600+ HP Dodge Viper, or a 535HP Datsun? Well, one of them has 1,200 Ft-lbs of torque and does 0–60 in 1.8 seconds and does a ¼-mile in 10.25 while the other has only 560 ft-lbs of torque, takes 3.4 seconds to hit 60MPH, and has a 10.9 ET.

The reason one horsepower does not equal the work that one horse can produce is because the term was corrupted somewhat by James Watt. James Watt marketed his improved steamengine by charging a royalty on the cost of coal saved over their old Newcomen steam engine. This didn’t work for people with no steam engine so the royalty was worked out on the savings of using the steam engine instead of horses. However when he worked out the power of a horsepower he rounded down slightly to give a slight benefit to the horse and so make it easier to sell the steam engine.

(I hope all that makes sense)

Thank You.