This monster should be large and the centerpiece of an adventure's / dungeon's climax. It is the Big Bad, the terror the party has been hearing rumors of, seeing clues of it's presence, and eager with anticipation and fear to finally face it. The exact stats (AC, HD, hitpoints, Damage, Immunities, etc.) are left up to you to determine. It should be tough though. The main feature is the Big Bad's counterattack.
Counter attack: Every time the Big Bad takes or is hit for damage it gets to immediately strike back. Attacking whoever just damaged it. Or, random opponent if source of damage is undefined or unattackable. This occurs in violation of any rules regarding initiative, number of attacks per round, etc. The Big Bad should have a melee and a ranged counterattack.
Ongoing threat: Instead of a normal attack the Big Bad (or it's environment, spawned minions, etc) drains opponents of some resource (hitpoints, stats, levels (ouch)). This is to make it tougher to work around / exploit limitations of counter attack.
Example Big Bad
This humungus blob of oozes, jellies, and puddings must be the source of all the things that have dripped on, corroded, slimed you in this dungeon. It's the Mother of all gooey things, end it's reign of gelatinous terror!
At all times the Blobinator is belching, spewing and otherwise projecting bits of ooze, slime, jellies, and puddings (use your favorites). Everyone and thing in the area of the Blobinator has a 50% of being hit by one of these each round. Figure damage/effects as appropriate for type of goo.
The Blobinator's primitive alien nervous system will respond to any attack with reactive counter attack. Melee and any target within 20' will be responded with a slam of a oozy pseudopod. To reach targets further afield the Blobinator can swing a pseudopod around like sling until the tip flies off to strike opponent. Critical hits result in random oozy thing's effect on target.
Each creature killed by the Blobinator will rise as a random oozy thing d4 rounds later.
Immune to fire, cold, acid, electricity, mind affects. Takes "normal" damage from holy water, alcohol. non-magic weapons have 25% chance of corroding, breaking, getting gooified each time they hit. The Blobinator can stick/climb along walls and ceilings.