There is an assumption being made here that magic has the same type of effect as physical phenomena.  IMHE, such is not the case.  Dumb beats tend to be immune from magical effects not specifically tailored to their mentality.  They even tend to extend their immunity to those around them.  For example, when was the last time you succeeded in using illusion to fool a horse.  If your horse doesn't believe that fire breathing bulldozer is there, you probably won't either.

Yeah, that's my conclusion as well.

I like this idea.

I'd also like to see a series of spear promotions that give bonuses vs mounted.

I could see horse riders having immunity to "pronicity" (the likelihood of being knocked prone) from footmen because of the angles involved, but i imagine enemy cavalry would be able to knock a rider off their horse.

Or just kill the damn horse.  If a cavalryman were to just wade around amongst enemy infantry during an epic conflict, I have no doubt the horse would fall sooner than later, no matter the training of the horse or the rider.

Mounts should be great at hit and run tactics, but extremely vulnerable to being flanked.