ezo
Hero
You had me up to "Spawn of the Elder gods".I’m genuinely intrigued now - in a game that runs from rats in the basement to dryads in the woods to gothic vampires to Mimics to Spawn of the Elder gods, what creatures make it "D&Dish"?

Seriously, the traditional core creatures (too numerous to list) all qualify to one degree or another. But creatures like Star Spawn or the Lonely and such ilk do not seem D&Dish to me. They seem more Cthulhu, not so much medieval fantasy adventure. Horror certainly has its place in D&D (mostly via the undead IMO), but many creatures do fall into a category of "beyond" D&Dish.
Certain settings, such as Spelljammer, will never be D&Dish to me, as well. If I want fantasy in space, there are other games for that.
We all have our preferences of course, and if I was in a game where the DM introduced the Starspawn or Sorrowspawn (?) as "new" creatures invading the "D&D realm" and the goal was to stop them, I could probably go with it provided the traditional creatures were there as well--even if the alien invaders were killing them off.
Not necessarily. It depends on what you mean by "monsters". There can certainly be strange and (relatively) unique creatures popping up in a game. The Underdark is a great setting for such. But a lot of it depends on how "explored" your world is. We're discovering new species on Earth all the time (granted less frequently), so encountering something "new" once in a while in a typical (for me) setting is fine.If the world itself seems sane and mundane and normal, monsters only exist in the dark corners -- the exact places where the insane, arcane and esoteric PCs are constantly poking their heads.
But where encountering the new happens more often and in regions where it doesn't make sense for such a creature to be unknown? No, that breaks immersion for me and lacks sense in world-building.
In a typical D&D game is a dragon a "monster" or a creature? In my games, they are still monsters (albeit sometimes good ones...).