First, I'd like to say that I have already been killed by the current unmanned horse AI....
Secondly, it is a nice idea... Horses that will rear players BUT without tweaking, this basically creates a attack horse (Like an attack dog) for a cavalry player.
Unless horses are given their own AI and a cav player would have to fight with the horse for control, this really wouldn't be logical or fair.
Sorry, but it is probably best to just leave horse AI as is...