With all the spell-slingers and magical swords about, it's amazing that more monsters are not immune to magic. Some perhaps with resistance -- some with odd forms like the pluses to a sword actually subtract from damage (even to the point of curing the monster) -- some with full immunity -- some that batten on it. Perhaps it's a hybrid monster held together by so much magic that magic just gets absorbed.
That would lead to rapid confusion as characters had to grab whatever was handy to make an improvised non-magical weapons, or figure out indirect effects -- perhaps the fireball could catch a building on fire, if only they could get the creature inside and keep it there? Or stone shape an unstable structure over it, and let it lapse?
A regular appearance would shift the dynamics of play in a game. A unique one would really have characters scrambling.