I rather like griffons as an encounter, because they are, statwise, essentially flying lions with AC as plate that come in unmanageable numbers of 2d6, but mostly care about eating your horses. A great calamity/opportunity to befall adventurers of any level venturing into the wilderness, really. Young griffons sell for 5000 gold and eggs for 2000, which is more reliable than the rarely-impressive treasure type C, but griffon adults, while fierce enough as is, fight to the death to protect their young.
AD&D Groaning Spirit (Banshee)
The spirit of an 'evil female elf' is, in the original monster manual, specifically called out as a 'very rare thing indeed.' With the later development of the drow, one might wonder if the underdark is absolutely infested with banshees. They are less menacing than spectres, dealing only 1d8 damage instead of level drain, but can do the famous death wail once per day, interestingly only being able to wail in darkness, and the death effect having a short range. Their appearance causes a save vs fear, so one could imagine them being a monster that appears, forces a party split due to saves vs fear causing fleeing, then the scream being used against whichever part of the party fled without light, with their impressive AC of 0 (20), typical immunity to nonmagic weapons, 50% magic resistance and 7 HD keeping them safe from quick destruction (save for the Exorcism, a 4th level cleric spell) and allowing them to mop up survivors.
Mythologically, it's interesting to note the origin entity (Bean Sidhe) was more of a lamenting spirit mourning the passing of families who were "true" irish, the Milesians, as opposed to english or norse or whoever else was about on the isle. Initially I assumed this degeneration from fairy-bestowed honor1 into an evil deathbringer spirit might be the usual christian demonization of local legends, but the idea of Milesians themselves come from christian pseudohistory, so this is probably just a case of muddled correlation into causation leading the myth to drift from warning of death, to being the cause of it.
Sunset Realm Griffon or Gryphon
|a Fault Griffon or Pelicat|
The noble gryphon is a chimeric hybrid the result of a political marriage between the King of Beasts, the Lion, and the King of Birds, the Eagle, the resulting beast a nearly unparalleled predator sure to sit at the top of nearly every food chain. Slightly more historically, they are the mounts of Mercian kings and noble knights, which gives them a certain appeal in some ages and places, and a certain symbolism of the rapaciousness of imperialism in others (especially Yuba, whose troop losses to wyverns in the Yuban crusades was the origin of the 'griffons love eating horses idea). As a very rough overview, Griffons were known in the late second age (once Elves rose up from the ashes of the Serpent Empire), were popularized in the 3rd age by elvish warriors and ensuing Witch-Knights, remained popular (though their numbers were reduced) through the 4th age, and were viewed more as just another monstrous animal in the 5th age, and one that was supplanted as the 'flying mount' of choice by Oroboron Goose Wyverns (or Gyverns) which, while equally vicious, were at least vegetarian.
However, in the height of the 4th age, the popularity of griffon mounts far exceeded the population endemic to mercia, and so the Beast Breeders of the Beast Battler circuit (of the Beast Islands, naturally) saw an opportunity, and so began the era of designer griffons. While the variety is too great to provide an exhaustive list,
some of the most notable were the Night Gryphons, owl/panther combos favored by assassin cults such as the Esoteric Order of Neuph, the Palace Gryphons, Parrot/Ocelot combos favored as gifts for bratty nobles who wanted gryphons suitable for admiring and entertainment moreso than warbeasts, and the Fault Griffon or Pelicat, a Pelican/Tiger combo that escaped and bred in the Fault after a shipwreck on the coast and was an absolute horror of a beast prone to swallowing people and young donkeys whole, and able to subsist of of sea-fishing in otherwise uninhabitable coastal areas and be just enough of a hassle to dragons that the two species preferred to avoid each other rather than battle for territory.
Sunset Realm Banshee
Elves are immortal in several ways, both biologically, and via reincarnation. However, this reincarnation requires a certain magical construct found at the heart of their cities, and of course, within the Iron Moon. If the soul is banned from returning to any city and able to resist the pull of the Iron Moon (typically thanks to death occurring well within the perimeter of the Daylands), it may linger just as any other soul might, and become a ghost. Given that being banned from reincarnating from EVERY available city means that one must be very near irredeemably unpleasant to be around, these spirits are often quite ill-tempered and dangerous. The theory that the name comes from a translated elvish court record that states '...from reincarnation, the council bans she who would slay without cause the mortal races for sport' could explain where the term came from.
The common misconception is that it is the scream of the banshee which kills, when in fact, it is simply the killing-spell the dead elf uses (typically at screaming volume) which kills. As such, there are banshees which use fireball to burn down any domicile built near the grave, banshees which strike people dead with lightning strikes, banshees that cause the heart to stop, and so on, depending on the favored spell the elf uses. As such, a proper silencing spell is good preparation, but plugging ones ears and going 'lallala' is not.
Cold iron may disrupt their form in death as it did in life, and serve as the DIY exorcism tool in a pinch, but the soul will almost certainly reform after disruption or escape rusted bondage sooner or later if destroyed via such generic means. Proper exorcism requires either petitioning an elvish settlement to open their reincarnation queue to the lost soul, or performing a ritual to banish the soul to the Iron Moon, or Elfland. Related note, no banshee will ever pass through an intersolar period without being collected by the Iron Moon unless located deep underground, so while the surface of Alvish ruins are typically free of this variety of restless dead, their depths, if any, do not have the same guarantee.
Banshees are not technically bound to the place of their death, but as they live under threat of being drawn into the Iron Moon, they typically stay nearby, only gradually expanding their area of influence over years of haunting and confirmation of lunar cycles, and typically staying nearest at night, when the Iron Moon could potentially draw nearer.