I Bet It Had A Name

What kind of horse was originally used to measure horsepower?  

I'm afraid to answer.  This is either a trivia question that I don't know the answer to, or a joke that I probably don't want the punchline to.  

Actually, it's neither.  I'm quite legitimately asking that question.  I'd like to know.   

So, it's pretty common knowledge that one horsepower is a unit of work equivalent to 746 watts.  Everyone also knows that the unit "Horsepower" was originally termed by James Watt himself.  At the time, I'm sure he was comparing the work something did to a horse, but that's somewhat a bogus unit of measure.  Every horse is different, and even a single horse's work output can change easily with any number of factors.  Has the horse been fed properly?  Has the horse been working all day?  Is the horse in a good mood?  Can the horse get a proper grip on the ground?  Is the horse's farrier competent?  (Look it up)  All of these things and more will greatly influence the amount of work a horse can perform.  So it seems a little wrong to have a basic unit of scientific measurement be based on this.  

Of course, now the concept of Horsepower has been refined, and it truly is a standard unit of measurement.   The whole 746 watts thing.  I'm just curious what type of horse under what conditions is equal to 746 watts.  Anyone know?