Oh, KRM, I don't disagree that they are capable of running at great speed and that the most recent descendents of the equine ancestors can run farther and faster (look at the feet!), just not that it's their primary function. If it were, I think they'd have a digestive system more similar to that of a cow or goat, who regurgitate their food later- they can literally eat and run, then bring it back up and actually chew it when they are in a safer location. Horses in the wild spend a huge amount of their time browsing (5 to 10 hours a day, I believe), which is hard to do when running at top speed ("What? Was that food? I don't know; musta missed it."). Running is an adaptation to predation; it's not the natural state when not in danger. The fact that they can tie up after great exertion to me indicates they're not meant to spend long periods of time at top speed. A predator is not going to pursue a prey animal for long-- heck, cheetahs are as fast as they come and they can't maintain the speed for long because they overheat.
I'm not saying they haven't evolved to be fast runners, just that their primary gait is walking and their primary behavior grazing. (And always happy to argue this with a fellow evolutionist.
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