The ever-great Topless Robot brings us a new D&D-related list, “The Most Evil (to Players) D&D Monsters!” It’s a good list. But there really is an unlimited number of “screw the players” monsters from 1e to draw from – the mimc, lurker above, trapper, goldbug, rot grub, throat leech… That’s the “old school” stuff that I hate.
I mean, how many of those monsters aren’t designed just to screw players? They all live on an exclusive diet of PCs and cash/jewelry/magic equipment. And all the oozes/slimes/puddings are an exercise in saying “Oh well here’s something that can only be damaged by one particular thing you probably don’t have…”
My favorite player-screw – the nilbog, a goblin variant that is healed by damage! That’ll show them! In a nonsensical way!
Geek Related 
I actually put a nilbog into Rise of the Runelords, with delightful results. See, one of the PCs was a hobgoblin who became the king of the Thistletop tribe after “Burnt Offerings”. He was originally a goblin, Klang, but after his name was written down by another goblin he lost it, and transformed into a hobgoblin.
So he goes off adventuring, now calling himself Neiman. Gets through “Skinsaw Murders” and “Hook Mountain Massacre”, then comes home to see how his tribe is doing. To his horror, all of the goblins are… reading! And practicing sustainable agriculture, and making goods suitable for trade! All of his goblin wives are wearing clothing and nurturing their children!
The dreadful culprit is Gnalk (spell it backwards), a nilbog who found a tribe with no leader to speak of and decided to make things better for everyone involved. His bizarro backwards-aura made all of the goblins do the opposite of their normal comedic-sociopath behavior… and also prevented Neiman from being able to murder the usurper and take back his kingdom, because he kept failing his Will saves.
Everybody, Neiman’s player included, thought it was hysterical.
Do the other players say “Newman!!!” when they see him coming?
Can’t say as they did, but the ranger’s retirement was to move in with him, so that way she would be happy and peaceful, instead of bloodthirsty and manic.