I think that (unless you are on it and can control it well) horses should flee from enemies, but you should have an option to command your horse to stay somewhere or follow you, and have the option to tie it to a tree or something.
It would be a well trained war horse that doesnt buck at the sight of a wolf, or try to run in the opposite direction of the fight.
Real war horses never did this, though they did get nervous and bash a friendly soldier who came too close his head in.
There would be no point to horses if they just added an extra annoyance factor to travel and fighting.