In pokemon MD, during the game the diglet and dugtrio always make mension of like walking, or "I couldn't feel my feet" or, something about swimming.
So, I think they look like a mole dragon-ish thing. I think.
Also, I always assumed that tangela's vines were like part of its body, not just covered. Like how we have skin. And like others have said, i think the same about shellder and cloyster.
Now, really, what does the head of a cubone really look like!??
I think cubone looks like a charmander or something like that. Now, how about Duskull? Behind the skull, what does it look like? I think it looks like gas or air, or something with one floating red eye.