Originally Posted by NorthernDancer
For example, if you knew how fast a horse runs and it's frontal area, could you quantify how much power that horse has like a car?
No, because the air resistance isn't the limiting factor, it's how fast the horse can move its limbs. If it could apply the same power in a more efficient way, it could go faster. Think of how much faster you can go riding a bike than running, even though you're producing the same power.
Or for working horses, can you use performance data to calculate how powerful a horse is relative to another for doing work? Has anyone done this?
Sure, that's how Watt (or whoever it was) came up with the definition of horsepower, back in the 1700s or whenever. The standard introductory physics texts all seem to have an illustration of a horse lifting a weight up a cliff. Today you'd probably use a treadmill or similar if you were being scientific, or measure oxygen consumption and convert that. Or just look at draft horse weight-pulling contests.