Predatory animals such as dogs and cats do one of two things if they perceive a threat. They either become submissive by laying down and cowering or they raise the hair on their back and turn to fight. Due to the major differences in behavior patterns, it is important to realize that horses have to be treated differently than dogs if we are going to get along with them effectively. Consider the differences between a puppy and a young foal. Puppies are real rough- housers.
Dogs like to be jumped at, patted roughly or scratched in an vigorous manner.
If a person got down on the barn floor with a foal in the same manner as with the young dog, the foal would be doing everything possible to escape from the situation. Once again, the foal perceives a threat and therefore uses the only tools it has available. It either shys from the situation in the first place or runs away if at all possible. Then the chase was on. I'll corner that nag.