In attempting to understand the horse, you must first realize that despite their present size, they have not always been the formidable critters that we know. The first horse (Eohippus, or "dawn horse" in Greek) was about the size of a Labrador, and tended to get eaten a lot by the local predators. Even as horses grew larger, they were still a staple on the diet of whatever preferred meat to grass. So the horse, over millions of years, learned a key lesson: Don't think, react. If something with a jawful of Bowie knives is bearing down on you, you do not pause to ratiocinate. You kick, bite, or run-instantly. This is why so large and powerful an animal is subject to strange fits and vapors, and is easily so spooked by so many seemingly harmless things. It has the mentality of a New York City subway rider who has learned to see mortal danger everywhere.