Tooth Beast

An ursine creature the size of a polar bear lumbers into view. Instead of fur, thousands of sharp teeth of various types and sizes stud its hide. Teeth of every kind cover its entire body, including its head, which has teeth for eyes. The raw flesh between the teeth is pink and has the moistness of gums. Despite its apparent blindness, the creature seems to sense the presence of prey, turning suddenly toward a creature that has passed and emitting a terrifying moan.

Source: Tome of Magic 3.5

Cool Factor: It is basically a giant bear with skin that has gum like consistency and teeth of various sizes clinging to it.It has no eyes and has a great sense of smell. This Tooth Beast is known to search the caverns of the Underdark for prey.
     The Tooth Beast is associated with the Vestige Dahlver-Nar, the tortured one. Vestiges are once powerful beings trapped between life and death. Dahlver-Nar is rumored to have been a Binder(kinda like magic user who makes pacts with powerful entities) who collected the teeth of other Vestiges. In the process he would rip his own teeth out and put another Vestiges tooth in it's place. By doing so he became a Vestige himself.

There is also a magic item called the Teeth of Dahlver-Nar which consist of the teeth of various other Vestiges. Each one grants different magical powers. The teeth are similar ioun stones. For example the tooth of the Vestige Paimon would grant you +4 enchancement bonus to dex. It does require you rip your teeth out to implant the item in your mouth.

Combat: The Tooth Beast has an interesting mix of mechanics behind it.

-The Tooth Beast is blind and uses blindsight to survey it's surrounding.

-It has the pounce and rake ability. Pounce in 3.X allows a creature to charge and get it's full complement of attacks off. In this case claw, claw, bite. Rake allows additional attacks when an enemy is grappled.

-The Tooth Beast has frightful moan. All within 30ft need to make a will save or be frightened for 1 round and shaken the round after. Once an opponent is under a fear effect the Tooth Beast has another ability called Sprint after Fear that allows it to increase it's movement speed to 60ft and charge for 120ft. The Tooth Beast initially only has a 10ft movement which is rather slow but once opponents are under any fear condition it gains battlefield mobility.

-It's Challenge Rating is 7.

Verdict: The Tooth Beast is interesting creature that can turn your standard cavern encounter into something unique. Maybe as the group is exploring the Underdark they notice dead bodies spread about. As they examine the bodies they find the lack of teeth quite disturbing. This leads to the creature causing this bodily thief.

Another option would be finding the teeth of the various Vestiges. Each of these otherwordly being carry unique powers. Maybe one of the Vault holding a powerful tooth it guarded by one of these beast. A whole campaign could be based around the Teeth of Dahlver-Nar.