BX Monsters A to Z: Displacer Beast

 It's definitely one of the weirdest creatures in the B/X bestiary. It's... the displacer beast, from the Expert Rules.

The displacer beast is described as resembling a black panther with six legs and a pair of sharp-edged tentacles sprouting from its shoulders, which always appears to be three feet from its actual location. A quick internet search for the origin of the monster reveals that it was inspired by a beast called the coeurl from a science fiction story titled Black Destroyer by author A.E. van Vogt. (I had never heard of either the story or author before, but I may have to check them out now.) Despite this pedigree, the displacer beast proper is one of the handful of creatures considered to be the intellectual property of Wizards of the Coast, and retroclones include it only under alternate names.

 The creature is quite robust, with an Armor Class of 4 and 6 Hit Dice. In addition to its very respectable AC, its displacement ability imposes a -2 penalty on all attack rolls against it. (Why not just make it AC 2? No idea.) It also gets a +2 bonus to all of its saving throws. The displacer beast attacks with its shoulder tentacles, dealing 2-8 points of damage each; a total potential damage of 16 points per round is nothing to sneeze at. Curiously, no alternative claw or bite attacks are listed; perhaps it always prefers lashing out with its tentacles so as to avoid directly grappling its prey, thereby effectively giving away its exact location. At 150' (50') movement rate, it's faster than a human, and its morale is a steady 8. Up to four of the creatures may be encountered together, which would likely prove a tough fight for at least mid-Expert level parties.

Displacer beasts are stated to be semi-intelligent, which means they could probably employ relatively advanced tactics, such as ambush, flanking, cornering, or herding prey into other hazards. Perhaps characters could even communicate with them on some rudimentary level, if they're not immediately hostile, with or without the aid of magic. They also might prove resistant to diversions like dropping food if they're determined instead to catch the party. (Dropping treasure, as for intelligent enemies, may prove more successful, as they do have Treasure Type D, implying purposeful collection of valuables.) Maybe they'd even accept bribes of shiny treasures in exchange for safe passage. They're also stated to hate blink dogs, and to attack them and anyone with them on sight. 

Other editions list displacer beasts as Lawful evil, so we might expect them to be Chaotic in the three-alignment scheme of B/X, but instead they're Neutral. Perhaps Cook and Marsh were a bit more sympathetic to the cat side in the eternal Dogs vs. Cats debate than was Gary Gygax. 

How displacer beasts fit into the ecology of a fantasy world is somewhat unclear. In a setting that includes sci-fi elements, they could easily be considered alien in origin. In a more traditional fantasy milieu, they might conceivably be portrayed as faerie-ish creatures, playfully malevolent fey-cats. Their intelligence and Neutral alignment might even make them viable companion creatures to some player-characters, though of course incompatible with canines, especially blink dogs. 

With no mythical or folklore analog with which to compare them, there's really not a whole lot of reason to modify the displacer beast at all. Their stats aren't wildly out of whack with other monsters (a tiger, relatively comparable in size and basic body form, has 6 HD too). I might make them effectively immune to missile attacks due to their displacement ability -- it's one thing to swing a weapon in a three-foot arc and hope to make contact, but quite another to try to hit a target with bow and arrow that could be anywhere in a three-foot radius around where it appears to be.