Hey,
Rgurskey's idea is the simplest,
Minifig, Head, Modified gets you most of these. There are also a couple of things under
Minifig, Headgear (wolfman, shark from Harry Potter, some of the Atlantis heads, Green Goblin mask, Islanders mask, Twilek headpiece, maybe the Batman cowl). The distinction is that first category are for those pieces that fit over the torso in place of a normal fig head and the second category are those pieces (along with your normal hats and hairs) that fit onto a normal fig head.
BTW, I think the oldest modified heads are probably Chewbacca and C3PO, going back to 2000.
Of course there are also normal heads that would be appropriate for purist monsters, like those from the Studios Monsters subtheme, the Monsters 4 game, etc.
Bruce