Almost a year ago, Aaron of Paladin in Citadel made a great case for using the literary ancestors of Displacer Beasts in a dungeon, and I've done just that, placing a Coeurl's lair on the first floor of my Mews of Methas megadungeon (as the keeper of this trap, actually). OSRIC and Microlite 74 have stats for the Coeurl, but here's stats for running the Coeurl in Swords & Wizardry. They're based off of the OD&D stats for the Displacer Beast, as received through Judges Guild's Ready Ref Sheets (since I don't have the PDFs), as well as Aaron's ideas and what I got reading "Black Destroyer".
Hit Dice: 6
Armor Class: 5 
Attacks: 2 tentacles (2d4 dmg)
Saving Throw: 11
Special: Greater-than-Human Intelligence, Create Darkness Ability, Magic (and Electro-Magnetic Energy) Resistance (80%)
Move: 15 (Metric Base Movement Rate: 5)
Challenge Level/XP: 10/1400
Terribly intelligent, always hungry for Id (measured in the square of a meal's hit dice), the Coeurl resembles a black panther with two tentacles - each tipped with seven fingers - rising from its shoulders, thin energy-manipulating tendrils for ears and forelegs twice as long as panthers on Earth. The Coeurl should be played as intelligently as possible, engaging a lower-level party in various schemes and tricks to get them to bring him creatures of higher hit dice than themselves, appealing to whatever motivation it thinks will works, including payment and justice. When a party a Coeurl forges a relationship with reaches a high enough level, the Coeurl will turn on them and attempt to kill and eat them, all while minimizing risk to itself. A Coeurl may hide its identity by creating a sphere of darkness around himself to a radius of 15' (~5 meters). As a super-intelligent being, an individual Coeurl may utilize magic or psionics at the referee's discretion.
And yes, before anyone says anything, the Coeurl is a nasty thing to place on the first level, but I don't intend for my players to fight it until much later in the campaign; if they do, well, they will have specifically chosen to do so and they also have two ray guns that might help (but, per "Black Destroyer, it has a very high resistance against energy weapons). I intend to use the Coeurl as Aaron suggests, attempting to employ the party in bringing higher hit dice monsters from lower in the dungeon to him for consumption. The party got the closest it's ever gotten to meeting the Coeurl last session and they may return that way again tomorrow, so we'll see what happens. I'll be excited whatever happens when they meet the Coeurl: if they lose, it will be entertaining, but if they win I'll get to roll to see whether a dragon takes note or not!
Finally, I have a question for my readers: in the Mews of Methas, the Coeurl's lair has two side-rooms. What would a Coeurl put in those side rooms?