Cerberus, Guardian of the Underworld
As your party approaches the black Gates of Hell, each spire creating the fence wreathed in flame, a black-furred beast some 15 feet in length from nose to tail drops heavily from his perch on an overlooking rock face. His form is entirely canine, appearing as an oversized wolf, ending in three massive heads, each one dripping with saliva and exuding the palpable stink of recently devoured flesh. With each repetitive pant from one of the three heads, a burst of fire is released. This, you realize, is surely the creature responsible for the charred bones you saw on your way here.
Cerberus is a uniquely advanced hellhound, possessed of three equally functional heads. He is charged with the protection of the gate of Hell, and will fight to the death even against foes who far outmatch him. This is because as the guardian of Hell, he is essentially unkillable, automatically reviving 24 hours after being slain. During this time, spirits may freely come and go from hell, usually causing havoc on the material plane during those 24 hours.