This, alone, will help us better understand the horse's behavioral nature. First, the horse is an animal of prey. It is preyed upon by other carnivorous animals. The horse is an herbivore. It does not hunt down other animals for food.
Humans are predators.
The horse will naturally shy away from humans unless taught that the human can be trusted as a gentle friend. Secondly, the horse lives by its flight or fight instinct. The first thing the horse knows to do when faced with danger is to flee. If it cannot flee, it will fight. The horse has to move its feet when frightened.