I had a pretty horrible one once. Only tangentially related to DF, but still...
I was (from a real-life first person perspective) in my house, which happened to be a fortress about 17 z-levels high and 34 deep. Suddenly a Hydra appeared, and this being my real life house apart from its odd dimensions, there was no way to actually fight it, though I didn't really think that far ahead in the dream anyway. I knew somehow that it hunted by scent and hid in a pile of blankets in a corner in one of the rooms on the third z-level, hoping it would leave before it killed and ate me. It walked past the room entrance, and somehow I knew that it smelled something odd, but wasn't sure what to do about it and so left for the lower levels. I was kind of seeing half from my perspective (folds of blanket) and half from the hydra's when it was nearby, and when it wasn't I still somehow knew about where it was. It chased around various of my relatives and pets, presumably killing them though I wasn't paying attention at the time, and I ran up to the fourteenth floor, since it would be less likely to climb all that way up. It did anyway, noticed something strange again, and this time decided to investigate, it tore up the curtains, bit me, tearing through the skin of my left arm, and I woke up. Aside from the obvious, the weirdest thing about the whole scenario was that when I finally saw the Hydra, it wasn't really a giant dragon with eight heads. It didn't have a real shape that I could see, apart from an enormous mouth which took up my whole field of view, though I know that it did have the other heads and a tail and forelegs and hind legs somehow. I got the impression that if we were living in DF, everything would look kind of like that: less like what it is, and more like an abstract representation of what it is. Of course, this also handily explains how it could fit through the door despite being thirty feet tall.