I think that the current horse bump damage is way too high, especially for armored horses - when a horse can bump you to death in 6 hits while wearing a total of 60 armor, having 27 str and 4 IF.
My suggestion is to nerf it by 30-50% in order to make it more of a teamwork-ability, giving your teammate a free hit instead of making horsebumps a killing machine.
EDIT:
as this is a general nerf, i cant give an accurate stat change suggestion, it would have to be decided horse by horse, this is mroe of a general "is horse bump dmg to high?"