Beyond a shadow of a doubt one of the most frightening monsters characters can encounter is the lowly Rust Monster. It isn't that it is deadly in and of itself. Heck it could care less about fleshy creatures, unless they have tasty metal with them!
In the weekly game, we recently had the final watch of our rest interrupted by two of these things galumphing into camp trying to devour every last bit of metal we had on us, which was quite a bit, some of it magically enhanced.
We dealt with them the best way we knew how . . . panic! The party thief grabbed a magic shield from one of the two clerics and high-tailed it to the opposite side of camp. The cleric on watch tried to face them down and lost one of the two plusses his armor previously had. My dwarf, suited in his skivvies and bearing his wyvern scale covered shield and an old hand axe interposed between the camp and the critters.
Martin the Ineluctable struck at them with a brace of magic missiles. That worked as it was first blood (technically) and they apparently didn't like the taste of his bitter bolts. The two fled our camp and the cleric with the wounded armor cast light on the tail of one as they dodged out of sight between some boulders.
The party then got our collective butts 200 feet higher up the cliff we camped at the bottom of. Thankfully those bastards don't fly.