After DF 2010?
Rust monster: Rots tissue made of metals. Toady actually talked about this, it was related to acid but I would bet it's the same thing.
Mind Flayer: A lot harder already. I think we'll have to wait for Magic, but I'm no pro modder.
Beholder: I wouldn't count it out. Multiple eyes would be easy although long (each eye needs to be defined individually), but until Magic only some of its powers would be possible to emulate. Still, it would sure be realistic enough for me.
But does the armor a creature wears count as "metal tissue?" To me, it seems like it does not, but alas my knowledge of the coming update is not 100%.
As for the beholder, I agree that many of its rays should be possible with the update, especially the fire and disintegration rays. The anti-magic cone of it's main eye would be useless anyway without other magical creatures around
But yeah, it's most likely possible even now to define the crazy body shapes of D&D monsters. It's just that a lot of them will lose a lot of their flavor without being able to use their signature abilities IMHO... Here's hoping that we'll eventually be able to add all of the great D&D creatures to DF, and our dwarven adventurers can get ambushed by packs of displacer beasts.
yay! So what are you thinking? Just DnD, or other fantasy games/books aswell, like could we have creatures from call of Cthulhu, DnD, Warcraft, ect. all in one game?
My knowledge of PnPs (as well as my RPG library) mostly includes D&D, CoC, WFRP, and Shadowrun, so I can help with creatures from those settings (though I'm not sure how relevant Shadowrun will be!)