How does a horse react when an enemy is approaching?

Horses are prey animals so if an ememy is approching they will try to escape by running away. If a horse hears something behind it and doesn't know what it is than it will probably whirl around to see what it is. However horses can be trained to be less "spookable" and some horses are just naterally less fearful than others. You could explode a bomb next to my pony(could not should!) and he wouln't move a muscle, but our horse Ivan spooks at leaves blowing across the grass.