Here's what I tried. I opened the console and clicked on my horse then entered modav <0>. But I got an error message saying parameter missing actor value. So I tried putting the horses id number before the command and got the same error. Am I doing this wrong or is it not even possible to remove the horse aggression this way?

not currently known, and i doupt that... we will be working on the horse for my realism/conversion mod, ill add this to the list that can be viewed here: http://www.thenexusforums.com/index.php?/topic/463687-realism-mod/page__p__3820554__fromsearch__1#entry3820554

i never been using any cheats in this game before, but if i remember correctly, in oblivion, you would open the console, click on the specific person/door/chest/object/wall or anything, and then type the command

These work when an actor is selected:
setav aggression 0
setav confidence 0

My horse stopped attacking after I changed aggression during combat.

I didn't try modav, maybe it only works on certain types of actor values.