It is only realistic that a horse knockdowns a much smaller creature such as the human.
To those who say knockdown is a cheap way to fight - sure, it is - but it's not hard to avoid getting hit if you are just aware of your surroundings which is not that hard.
Every attempt to bump someone means risking your horse and your life.
Did I mention the down block? It always works, no matter if the thrusting lance aims for your feet or your head. How cheap isn't that?
Also, if you kill a horse running straight at you just 1cm before it is about to hit you, you don't get hit at all...
"Only a trained horse would charge a group of footmen - trained - and armoured (No living creature will want to get hurt - unless very strictly trained - or not at all)"
Good point. I agree. But how would you train it to obey? You can't pick the riding skill obviously but if the CHA and INT attributes would be available you could use one of those for it maybe...
I actually made a post about the free will of horses recently
http://forum.c-rpg.net/index.php/topic,6617.msg104279.html#msg104279