16 Favorite Aberration
The nomenclature here is a bit puzzling, and I suspect that players of later editions are more conversant with the rigid stratification of creature types and templates, but I take an aberration is a monster whose very alienness marks it as monstrous. In that case, although the otyugh and aboleth are strong contenders, I’d have to go with the unsung doppelganger.
Dopplegangers aren’t that tough head on, but add in their shapeshifting, the magic items they’ve stolen or claimed from victims, and the general mayhem they can cause, and they have serious potential to wreck an entire gameworld or turn it to their whims.
DMing a doppelganger sometime soon? Go and watch John Carpenter’s The Thing, then read The Things by David Watts. That’s the level of headgame mindfuckedness you should be aiming for.
17 Favorite Animal/Vermin
Animal or vermin, which is it? Once again, my D&D knowledge roll is a failure. I’ll go with animal and say I always liked the idea of cave oxen, herds of musk ox-like herbivores roaming the ginourmous caverns of the underdark. Adventurers are prone to see them as a food source, that is until a stampede of these creatures cause a TPK or something near it.
18 Favorite Immortal/Outsider
I have no idea what an outsider is (Ralph Macchio’s Pony-Boy comes to mind), but as for immortals, there was a monster that looked like a blue-skinned Mr. T from 2e D&D that I loved. I forget the name, but they were like the cosmic merchants called the Devourers from the old Fafhrd & Grey Mouser tale ‘The Bazaar of the Bizarre.”
Although some of my friends didn’t like idea of some mystic giants setting up stores and cafes in a dungeon as it was ‘unrealistic’, I think unrealistic is a critique that falls pretty flat in a game about dungeons and freakin’ dragons. It was also a fun way to have a ‘save point/store’ in the middle of a tediously long delve.