Leech, Leopard, Leprechaun, Leucrotta

 AD&D Giant Leech
Sneaky creatures that suck blood with an anaesthetic bite, with only  1% chance of detection until the afflicted is reduced to half HP, slowly sucking blood at the rate of 1HP per HD of the leech (HD ranging from 1-4). They are very slow, so if they fail to attach via initial attack they are unlikely to pose much threat.They can be killed with salt, and attack people camping near leech infested waters at night. Unlike real life leeches, they also spread disease.

I like the idea of warding off leeches with a circle of salt while camping at night in wet areas, or carrying bags to kill the big 4HD varieties, adding procedures to check each other for leeches, etc etc. Similar to green slime adding a 'check the ceiling' for adventuring procedure, I think leeches are a similarly excellent complication to sodden areas.

AD&D Leopard
The endless slight variation in the claw/claw/bite routine, # appearing, and HD of the big cats, each with their own entry, continues to confound me. Various other highly similar creatures got lumped together in a single statblock (Horses, beetles, etc) so I don't know why the big cats didn't get the same treatment.

AD&D Leprechaun
These creatures are fragile fey that will attempt to steal valuables 75% of the time, with a 75% success rate, then flee, with a 25% chance to drop the item if closely pursued. With treasure type F, they are likely to have decent treasure themeselves, so if the players can get around an advanced movespeed, invisibility, non-living objects polymorphed into other inamiate objects, illusions, and ventriloquism (as well as an immunity to being surprised due to good hearing and 80% magic resistance), perhaps by the noted 'fondness for wine,' stealing a leprechaun's pot of gold, so to speak, could be a much safer endeavor than dragon-hunting, but a similarly lucrative one.

While rather more derived with twee Irish tourist trappings (and possibly nasty 19th century stereotypes) than any solid mythological source material, they are a pretty good challenge I think, requiring creative problem solving to both learn the location of the treasure and avoid being tricked by the monster. Better yet, they are often active antagonists who will steal something from those they encounter  with no reason beyond that of mischief, which makes the question of 'is it ethical to rob leprechauns of their treasure' likely to be a clear cut case of retrieving stolen goods from a supernatural trickster spirit, rather than extortion and robbery of an innocent person, and can jumpstart the players into action.

AD&D Leucrotta
While supposedly so ugly that other creatures cannot bear the sight of it, I always thought the art and description didn't really live up to this. A stag body, a lion tail, and a badger head? Just another furry animal, really. Later editions tried to uglify them up by making them more monstrous

It can imitate the voice of people, to lure them into bite range, but I don't think this adds up. It lives in 'deserted and desolate places' and runs almost as fast as a horse, and has a 3d6 bite attack, so it is perfectly capable of running down humans who don't have a speedy horse and devouring them without the need for subterfuge. It also gets 2 1d6 kick attacks when it flees, perfect for 'hit and run' rather than ambushing. So why the need for a lure?

The Forgotten Realms wiki mentions various tactics used by them, some of which were 'neat' but didn't address my basic concern that the statblock suggest a hit-and-run pursuit predator, while the described tactics and mimicry suggest a trap or ambush predator. From all this, I think the best way to 'solve' the problem is to put them not in desolate wastelands, but near humans, lairing in ruins (which may have mechanical traps to lure humans into), along roads (luring humans off the path to devour so as to leave less trace of the struggle) perhaps by rivers (where disarming and dismounting is likely), and certainly near hunting grounds (where their footprints, indistinguishable from that of stags, will draw hunters to them).  Additional explanation for their behavior could be that they do not have particularly good senses- no keen sight to spy prey across open plains, no strong nose to follow trails, and hearing good enough to imitate speech but not good enough to hear prey attempting to be stealthy. As such, it becomes important to lure humans near, ideally dismounting from any horses, and lead them into areas where there are no easy escape routes or places to hide.

Sunset Realm Leeches
Giant leeches come about when a leech sucks the blood of a giant, naturally enough, growing larger and more of a threat. As becoming larger is common worldwide, so too are the threats of giant leeches in wet places.

Frog-witches of the Bog of the Canal know that leeches are good for drawing out magic through the blood. A leech that feeds on someone with a potion effect will gain it for themselves, and various disease spirits and poisons can be drawn out of the body via the application of leeches to key points as a healing method. While some find it disgusting, potions-leeches, to be consumed later, are a form of alchemical reclamation and recycling far more available in the bog than fancy glass alembics and such.

In Vint-Savoth, leeches are used to draw out corruption of the blood from the Blood Moon by the Sanguinaries of the Sanguine Church. Such corrupt leeches are disposed of, but those that survive may grow into horrible sewer beasts that squirm through the blood-gutters, consuming the fetid flow of disposed blood that trickles down to them. While such monsters may help control the population of blood-corrupted rats, they are ranked highly for disposal, as new strains of the Moon Scourge may develop rapidly in their churning, blood-filled guts due to their indiscriminate feeding.

Sunset Realm Leopard/Jaguar
Jaguars/Leopards are the big cats of Yuba, thought to be the reincarnated souls of Yubans of questionably morality who ALSO didn't really like dogs, and so returned as cats instead. It is said that the spots are sins that prevent the soul from moving on, and upon death the soul splits, the spots left by past misdeeds falling off, and the clean parts reforming into the original soul, now free of wickedness and able to join a peaceful afterlife and leave the world behind. As for the spots, they combine into stripes and slither around, seeking to attach themselves to wicked cats that are becoming tigers. Unlike the prides of lions or the solitary tigers, the spotted big cats work in pairs, pairs being assigned such that their spot/unspotted skin ratio comes out to 50-50 when accounting for both of them.

Sunset realm cat taxonomy is rather different than real life, caring only whether they be striped, spotted, or solidly colored.

Sunset Realm Leprechaun
While touched on somewhat here, this variant of Fairy, an artificial spell-construct is not only in charge of fake gold. Some leprechauns, serving long-vanished alf-creators who tasked them with collecting taxes and managing treasure vaults, still function, attempting to retrieve treasure from humans dwelling in lands that, long ago, were part of the elvish empire. While largely harmless alone, they will invariably be working with other fairies or shadow-goblins to aid in guarding and enriching the stale hoards of their masters. Since said masters, having been locked up in the Iron Moon or vanished into the dreams of ice or trees, will not be coming back, the risk of serious retributive action from the fey is low and raiding leprechaun treasure ranges from 'archaeological expedition' to 'yearly sport' depending on location and relation.

Sunset Realm Leucrotta
I think the same can be accomplished by any animal that learns Commonid and chooses to be villainous, so I will instead utilize ventriloquist goats that lure people to cliff edges ("Is anyone there? Help me up! I can't hold on much longer!") and butt them off when they go to investigate the cliff edge.

I don't know about your players, but mine are likely to think weird human noises in suspicious places are a trap. If they see a beastie like this though they'd probably walk right up to give it pets and snacks.

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