Lamia, Lammasu, Lamprey, Larva

 AD&D Lamia
Lamia are bulky, leonine, centauroid creatures with 9HD and armor as plate, who drain 1 point of wisdom permanently on hit, and have a handful of spells- charm person, mirror image, suggestion, and illusion. Characters drained to 3 or less wisdom do as the lamia says, but as no mention of lamia keeping such simps around is made, it seems like they only become food rather than slaves. It is also unclear if the wisdom drain works on everything, or just on prey species.

While it has the usual idea of 'if you are alone and fail a save you die,' it has potential to use the environment against players via its high movement speed, illusion to cover hazards, so a sort of determined 'hit and run' monster causing permanent character damage could make for a good mix of infuriating and terrifying. Still, I think it would think it would have more potential to be an interesting encounter with bewitched people and monsters assisting it.

AD&D Lammasu
Another force of good derived from assyrian myth and clumsily jammed into D&D cosmology. Statwise, it's the usual uninspired mash the forces of good have- 8th level cleric casting, it's a flying lion, invisibility, dimension door, aura of protection from evil, better healing, 1/10 chance to be able to speak a Holy Word, some magic resist. A likely strategy for them to employ would be hit and run in surprise invisibility attacks, then flying or teleporting away to heal each others wounds efficiently before returning. Add in some debilitating effects like Hold Person or reversed spells and I think they'd be quite effective at whittling down the forces of evil, and despite my dislike of the lack of original flavor, it could actually be quite an interesting tactical problem to deal with a pack of these creatures, and they'd make excellent support for other beings as well.

One thing I dislike about this entry is that their common spell-list is not provided as it is in, say, 3.5, making them a bit annoying to use on the spot, as one must whip up a 8th level clerics spell list. One also wonders at the disparity between Good and Evil in which infernal Evils can only be hit by magical weapons, but many Good celestial beings can be brought low by an ambush of bandits with slings and a surprise round.

AD&D Lamprey
Self explanatory really- simply more giant than real world counterparts, they suck blood. I enjoy their method of attack differs from the norm- they first attach for a small amount of damage, then drain a fixed rate of HP each round thereafter- 2 for the 1+2 HD variants, and 10 for the 5 HD massive variants. That makes it so the players can prioritize removing attached lampreys and defend each other, and provides a simple, obvious threat and counterplay.

AD&D Larva
While mechanically they are nothing more than demonically themed 1HD cannon-fodder, the lore of these creatures is pretty good. 'The most selfishly Evil' souls becoming human-faced worms, which are then turned to Quasits or Imps, or used as a form of barter by liches and night hags is quite flavorful.

Sunset Realm Lamia
Yg, mother of serpents, offers wisdom and snakeliness to those who sacrifice their legs to her and become a serpent. Lamia are the first stage of Serious Devotion to Yg, humans whose legs have been replaced by the tail of a snake, or the offspring thereof. The next stage of serpentine enlightenment is the Naga, a snake with a human head, after which there's nothing left but to become a full snake, though the combination of grueling divine trials of worthiness and the appeal of having thumbs means that most Yg-cults plateau at the Lamia stage. Medusa are a different set of requirements entirely, though there is theoretically nothing stopping a nagadusa or a snakedusa from being a thing. It could be the honor of being a repository of forbidden knowledge also contains secrets that mean medusa do not strive to become more snakelike, or more complete snakes are spared the burden of this knowledge.

In any case, rather than mysterious monsters like medusa, Lamia are a fairly common kind of person to be found at the intersection of Yg serpent cults and human society. Though they do tend towards occult wisdom and bookishness on account of the pact with their god, the immense strength of a snake tail that big makes them rather good wrestlers and swimmers as well. Subsequent generations will tend towards the human part becoming snakier as well- scales, fangs, venom, forked tongues, slit eyes, cobra hoods instead of hair, eggs instead of live birth, etc etc, as Yg slowly warms up to them and blessings accumulate over generations.

being lignified by the Lumarian curse "Snakes to Sticks" results in a non-zero amount of lamia trapped in awkward locales with the upper torso of a human and the lower half of a large vaguely serpentine log. Such are the perils in being involved in a divine war of knowledge (Yg) vs secrecy (Lumar)

Sunset Realm Lammasu
Nah, though 'winged beast with a human face' is a fairly common motif among divine servitors.

Source unknown

Sunset Realm Lamprey
Though no fault of the fish, lamprey are most known not for their own form, but for the Oartia bloodline of vampires. Originally a kind of curse derived from lamprey souls used in mad experiments beneath the sea in the laboratories of the Ningen, the giant Ningen lamprey-vampires spread it to humans somehow or another. Such vampires are smooth, flexible, amphibious, and faceless, typically concealing their terrifying toothy maw beneath masks, long hair, or giant hats. Like most animal-type vampires (as opposed to undead ones) the Oartia bloodline is found most frequently in Vint-Savoth, as the accumulation of accursed moon-blood through hemophagy grants monstrous powers. While the Oartia may have originally been merely afflicted humans who could have lived on donated blood, the creeping corruption accumulated through years of bloodsucking has surely switched their categorization to that of monsters, driven from the cities but still ruling in secluded coastal or swamp castles, their curse spreading to other animals as well in cases of long-turn habitation.

Sunset Realm Larva
Rather too specific to D&D cosmology