The Narcissus-, or Undesirable-, Peacock

Narcissus Peacocks, also known as Undesirable Peacocks (a perhaps over-literal translation from the Tsolyáni) at first appear to be ordinary male birds of the species cristatus or muticus. Once the animal spreads its fan, however, a baleful disparity is revealed. Reports of precisely what a Narcissus Peacock's tailfeathers look like when flared differ wildly--of the few who have seen the sight and lived, none were talented wildife painters.

Popular superstition holds that the "eyes" of the terrible bird's feathers change and warp to resemble the victim's own eyes in some inscrutable way (thus the name) --while others say the "eyes" act as scrying devices for forces unknown. Some hold that the fan contains no "eyes" at all, but instead displays, in a perfect feather-mosaic--the portrait of a god.

What is certain is that when the animal opens its fan, creatures of weak will stand transfixed and fascinated by whatever it is they see there, and are willing to stare into the open fan for hours while the peacock devours them.

Luckily, the animals are extremely rare. They are occasionally bred as pets for rakshasas, Melnibonéans, and certain of the more decadent Elvish courts. Medusae--who are immune to the fans' enchantment--build on the birds' natural affinity for shining objects to train the beasts to attack mirrors on sight.

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Crunch:

Physically it's just a 1 hit-die bird, though the beak and claws should do d6 damage and anyone seeing the spread tail requires a saving throw to avoid being transfixed. If you're feeling generous or have a low-level party, allow any victim to re-save every time s/he takes damage. Killing the animal or just damaging the tail feathers removes the effect. The feathers are probably worth a lot of money to the right wizard and the low-level version of the beast itself is worth 75 xp in AD&D.
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Slight tangent about inventing this monster:

The peacocks came about just before a session where:
-The party already knew there was a medusa in the dungeon, and
-They knew where its lair was
I realized that the next session could very well turn out very boring, because although they make great villains (they are intelligent, luxury-loving, and have a ready-made reason to be isolated and hate the world) a medusa is kind of difficult to run as a "boss". While a medusa is a relatively high-level monster, as soon as a party knows it's going after a medusa, it becomes a low-level monster. A medusa out of nowhere is a challenge so huge it's almost cruel, a medusa you are actively hunting is just like any other monster, only you can kill it instantly with clever preparation and a mirror. Either way, the encounter would be short.

The other problem was that while knowing you are hunting a certain specific kind of dangerous monster definitely makes the session more ominous (which is good), it somewhat kills the mystery (which is bad). So I decided that the first thing they found in the medusa's hall of statues had to not be a medusa. As soon as that happens the players realize they're not out of the woods yet. Who knows what other pets she's got in there....

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In play:

Almost had a TPK when these showed up. Everybody but the paladin (8 PCs) failed to save. Then the paladin went to kill the peacock, rolled a one and hit a transfixed fighter. The fighter woke up, tried to kill the peacock, rolled a one and hit himself. Then they started hearing footsteps and hissing in the next room...

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ADDENDUM:

Ragnorakk, I accidentally deleted your comment while trying to fix the...long story, anyway, I read it.