[paisjpq]

thats a good question and of course more difficult to answer about a horse than a car. (I think, but I don't know a lot about cars so I'm guessing).
yes, some horses are natural fighters, they will dig deeper and deeper to try to either pass horses or not let anyone pass them (whichever the case may be). Watch the ears--Funny Cide is a good example, he will lay his ears flat and dig in down the stretch.
Others definately are lazy and run better for an aggressive jock--the problem is that many horses will sulk or quit if you get after them too often or too hard. This is where a good jock is invaluable as he can find the line between too much and not enough. That's the short answer.