Here's something really nasty I came up with for a high-lvl/high-power campaign long ago, slightly re-tooled.
The VORPION is an intelligent Gnarthropod resembling a giant scorpion, with one pincer larger than the other, and a wicked pronged barb instead of a stinger. It's metallic, glossy black carapace is crisscrossed with scarlet markings resembling a network of veins, and its limbs are ringed with crimson bristles. This creature can Detect Magic, will attempt to attack Spellcasters in preference to all others, and likes to collect Magic Items... and skulls. The larger pincer is the Crushing Claw, while the smaller is the even deadlier Vorpal Claw. The Crusher does 2d8 damage, and once a hit is scored, damage is automatic each rnd (Bend Bars to escape). The Vorpal Claw does 2d6 and decapitates opponents on a natural 20 TH, and if this attack is used on a foe already in the clutches of the Crusher, only an 18 or better is req to sever the head. Note that DEX bonus to AC is negated while in the grip of the Crusher. The tail barb impales for 2d4, doing an auto 1d4 the next rnd as it is withdrawn. The tail is controlled by a secondary, lesser brain, which can still function independently if the main brain is affected/influenced somehow. Although it does not normally attack with its mandibles, it will do so if it has no other choice, for 1d4 dam. If a head is severed, things get really creepy, as the tail barb ceases to attack and instead impales the severed head and swings it aloft like a lantern. The head's eyes light up bloody red, and the Vorpion then employs any Mental Powers/Spells/Knowledge stored in it's memory against any remaining attackers, taunting them in the voice of their former compatriot! (Freak 'em out: have one appear with an already skewered head, cast a nasty Spell, toss the drained head, and advance seeking another!)
# appearing; 1 for God's sake; MV 12"; AC 3; HD 7; AT 3; Crusher 2d8+ crush, Vorpy 2d6+ snip, Tail Barb 2d4+ impale; (Spell use); Size 10' long; and as if all this isn't enough, the Vorpion takes only 1HP/die of Electrical damage (no save), just 'cause; XP; your game; Treasure; see XP, but give 'em a break!
Vorpion carcasses really should be worth something on the Alchemical/Spell research market; possibly their secondary brains are used as an ingredient in some arcane brew, their eyes (four pairs of increasing size -oh, they live underground, so Infravision!) are used in Potions, barbs could be made into (possibly magical) spears, and I'm thinking their claws might be fashioned into cool-looking shields, either small or large. (NOT Vorpal-edged shields, though!)
UPDATE: I just figured out why my battered notes on this Party Killin Machine read "Can employ Wands, etc, but not weapons"... the original draft did not have a tail spike, but rather an oversized human-like hand, which performed the same functions as above. It was orig created as some sort of Wizard-hunting beast...