http://www.horsequest.com/journal/educate/fbell.html
For over 9,000 years humans have been hopping onto the backs of horses with the intention of going somewhere. We have been doing it so long that we have forgotten that what seems natural for us is totally unnatural for the horse. To put matters in context, consider that the horse evolved over millions of years to become the exquisite creature that it is. Through the process of natural selection, those traits which enhanced the horse's chances of surviving long enough to reproduce were favored. These were the traits which enabled the horse to avoid the predatory animals which stalked him, carnivorous predators such as the big cats, wolf packs, and man.
Since the horse's predators were among the fastest creatures on the planet and in the case of wolves and man, they were also the smartest—most cunning predators, the horse needed incredible instincts to survive. Their survival is proof thatthey posses these instincts.
To counter the stealthy stalking of their enemies, horses developed one the most sensitive alarm systems in nature. The slightest perception of an unnatural movement can trigger the flight response which for countless millennia enabled the horse to escape the stealthiest cats, and cunningest wolves, and smartest humans.
The horse can smell a carnivore at great distances. The horse can hear disturbances to the natural order of sound that humans cannot hear. The horse can see movement imperceptible to many other species, including humans—and can do so in the dark of night. The horse is an exquisite sensory system of nerves which culminate into a hair-trigger alarm system,
which when activated, releases an explosion of energy channeled into an instantaneous flight for survival.
The horse is naturally fearful, and for millions of years he has had every reason to be. And he has lost almost none of this instinct in the 90 odd centuries that he has been associating with humans.
Selective breeding has done a great deal to change the appearance of the horse in the past several thousand years, but it has changed the essence of the horse very little.