Here is an example but i have seen many like it
That's a matter of interpretation. To me, the horse doesn't look scared.
Its head is up, its ears are forward but easy, it isn't rolling its eyes or staring at something frightening, its tail is lifted, not down. No, the crupper under its tail isn't responsible for that: it can perfectly well tuck its tail down over the crupper.
Its mouth is open, but you will see this in a lot of art because the bridle is, by our standards, a bit short, pulling the bit too tight, so the horse opens its mouth to relieve the pressure. Ann Hyland, in her The Warhorse books, notes this.
So your presumption that the horses look/are scared is an initial problem in answering this.
On the other hand, Training the Roman Cavalry: From Arrian's Ars Tactica by Ann Hyland reaches the conclusion that a good deal of the training, riding in groups with imitation combat, is to restore the faith of cavalry horses in their riders after the horses have been injured in combat. The horse has to learn to not shy, for fear of being hurt again, on approaching other horses head-on, or just riding into the battle in formation.
That's actually not a rear- it's a levade:
Note how the horse is loading his weight on his hind legs. Compare that to a rear:
http://www.thinkequus.com/wp-content/uploads/2010/04/rearing21.jpg
In the rear, the horse is throwing his weight up to try and unseat the rider. In the levade, the horse is shifting his weight on to his back legs and is lifting his weight up. While a rear shows a lack of control, the levade shows expert and masterful control.
The painting shows that Napoleon is in complete control of his horse, despite the horse being spirited.
