Here's one of my all-time favorites.
VILTCH
FREQUENCY: Rare
NO. APPEARING: 1-20
ARMOR CLASS: -4
MOVE: 21"
HIT DICE: 5 + 2
% IN LAIR: Nil
TREASURE TYPE: Nil
NO. OF ATTACKS: 2 claws and 1 bite
DAMAGE/ATTACK: 2-5/2-5/2-8
SPECIAL ATTACKS: Minor poison; spells
SPECIAL DEFENSES: Spells; never surprised; various immunities & resistances
MAGIC RESISTANCE: 40%
INTELLIGENCE: Average
ALIGNMENT: Chaotic neutral or chaotic evil
SIZE: M (5' tall,)
PSIONIC ABILITY: Nil
Attack/Defense Modes: Nil
LEVEL/X.P. VALUE: VI/575 + 6/hp
Vitches are hateful creatures from the plane of Pandemonium, inclined to malign destruction and disorder. They are skilled vandals, ripping open wineskins, spoiling food, biting through ropes and straps, tormenting domestic animals, smashing wood, and generally creating havoc wherever they go. The more beautiful or orderly an object or an arrangement is, the greater their determination to destroy it. Though they can inflict fatal wounds, viltches prefer to cause mere scratches and nicks, concentrating on various random acts of hooliganism. They seek out weak prey and avoid direct confrontation with powerful creatures.
The following powers are available to a viltch, usable one at a time, once per round: detect traps, dimension door, shatter (three times per day), teleport (once per day), trip (three times per day), warp wood (three times per day), and gate (30% chance of 1-20 more viltches from Pandemonium). A viltch is immune to electrical damage, psionic attacks and powers, and poisons or gases of any sort. It takes half damage from cold or fire. Viltches have both infravision and ultravision out to 90'.
Viltches are never surprised and always gain +2 on initiative rolls because of their speed. They have the abilities of 12th-level thieves in picking pockets, opening locks, removing traps, and climbing walls. A viltch can dodge non-magical missiles if it saves vs. petrification. The venomous bite of a viltch causes pain in a live victim (-3 "to hit" and no chance of casting spells or attacking psionically) for 1-6 turns.
A viltch resembles a mandrill with matted and disheveled fur, and is a dirty gray color. Its muzzle is blue-gray. It has a mane of darker color, and its eyes burn a baleful yellow. A viltch has only three legs: a single leg in back, and two in front. It seems not to be handicapped by this arrangement.
Originally appeared in Dragon Magazine #94 (1985).