If I wanted to picture a horse capable of delivering one HP over a work day, what kind of horse am I looking at? A thoroughbred, a draft horse, a glue factory reject, a non-existent superhorse?
The wiki entry states that the current measure might be more than an actual horse can deliver for any useful length of time.
What would be the actual power rating of the common types of modern horses? Wiki also mentions a peak performance of almost 15 HP from a horse but doesn’t go into much detail as to what kind of horse, doing what and for how long?
What’s the power output of a racing horse during a race distance (which varies, I know)?
In short, tie the mechanical unit to actual animal performance in any way you can.