Adaptational Badass: In the first edition, a demilich -while far from being harmless or easy to destroy- was a pathetic creature that spent most of its time doing nothing and was really only dangerous when touched. In second edition, however, a demilich is simply a more powerful version of a normal lich.
Adaptational Wimp: In 5E, demiliches go back to being comparatively weak, being liches that have decayed after not feeding their phylacteries' souls, though some, notably those connected to Acecerak, are given altered abilities that make them much more powerful.
Affably Evil: Several examples can be found across numerous sourcebooks. Particular examples include a few of the Lords of Hell (Dispater, Belial, Fierna, Glasya, and Asmodeus), a very few demon lords (Grazz't, Dagon, Malcanthet), some gods, and a smattering of various beings.
A God Am I: The Forsaken, a race of humanoids who seek to gain immortality by devouring divine power sources, whether they be artifacts or living creatures.
Alien Hair: Genasi and shardmind can have crystals instead of hair, Wilden have spikes.
All Animals Are Dogs: To a lesser extent than most examples: a few of the tricks one can teach an animal using the Handle Animal skill, such as "fetch" and "heel", are generally associated with domestic dogs in the real world.
Surprisingly averted in the Dark Sun setting with Muls (half-dwarves), which are larger than either parent (much like real-life ligers). The sourcebook even lampshades it by noting that one might expect a half-dwarf to be exactly between a dwarf and a human in size.
All Trolls Are Different: The trolls in D&D are actually pretty consistent in their large size, low intelligence, savage demeanor, regenerative powers and distinctive pronounced noses. There are a few variations on the theme, though, from the huge mountain trolls to the small forest trolls to the sea trolls.
Always Chaotic Evil: Trope Namer. Lots of Creatures have destructive Alignments hard coded into their being (though mostly just dragons, werewolves, and demons). Many monstrous races have reputations of this, but reading the stat blocks, they are simply listed as "Usually Chaotic Evil".
Antlion Monster: The giant ant lion first appeared in Advanced Dungeons & Dragons in the Monster Manual II. It was Large (larger than human) size and acted just like the Real Life version — digging a pit and waiting for victims to fall in. Once they did so it burst out and grabbed them with its huge mandibles, crushing them until they died.
The Artifact: For older D&D settings, at least, the entire Always Chaotic Evil trope. Newer settings like Eberron, and Iron Kingdoms tend to avoid it and base most conflicts on Grey and Gray Morality, prejudice, and cultural differences. Older settings have attempted to change many of the races to USUALLY Chaotic Evil (Drizzt being the best known example of a subversion), but it still leaves the implication that the overwhelming majority of these races are evil.
Ascended Demon: This "can" happen (a notable example on the Wizards site is a succubus who fell in love with an angel and is fighting her inner nature to be a Paladin), but it's very difficult for fiends — beings whose very being is composed of Evil — to fight against that nature to take upon a Good alignment.
Attack Its Weak Point: In the Melnibonean Mythos section of the 1st Edition Deities and Demigods Cyclopedia, the demon lord Pyaray can't be killed until the diamond-hard pulsing blue gem in his head is crushed.
Badass Beard: Male dwarves, obviously, but 3.5 Spell Compendium includes a spell called "Silverbeard" which gives you a+2 sacred bonus to Armor Class and a +2 circumstance bonus on Diplomacy checks regarding dwarves.
Badass Normal: Most martial characters in 4th edition, and high-level fighters, rogues, barbarians and so forth in earlier editions, are able to function pretty well despite being basically muscular individuals with absolutely no magical abilities.
The boggart is the immature form of a Will-o'-Wisp. It is a Shapeshifter that can change its form between that of a small demihuman/humanoid and that of a will-o'-wisp (ball of light).
Most types of Eladrin (fairy-like outsiders from the Chaotic Good planes) can turn into a ball of light, depriving them of most of their abilities but allowing them to become incorporeal, fly and shoot energy blasts.
Basilisk and Cockatrice: D&D split this creature into two versions. The basilisk is a large lizard-like creature. Meeting its gaze causes the viewer to turn to stone. The cockatrice is a chicken-like creature: being hit by its beak causes petrifaction.
Beak Attack: In 1st and 2nd Edition Advanced D&D, many monsters (both bird-like and non-birdlike) have beak attacks that do enough damage to kill ordinary people (who have 1-6 Hit Points) with one blow. Some of these monsters and the number of Hit Points of damage their beaks do:
Asmodeus for the Devils. Demogorgon, Orcus, and Graz'zt for the Demons. In 4th edition, the dark god Tharizdun shows signs of being the ultimate Big Bad due to being opposed by literally every other god and being directly responsible for the creation of the Abyss, and the Player's Handbook 3 hints that he's also responsible for the Far Realm's incursion into the material world.
Elder Evils was basically a book that detailed several of these, and how to play them out over the course of an entire 20-level campaign; the campaign was basically capped off by confronting said evil.
Big Eater: Giants have appropriately giant-sized appetites, but Hill Giants especially, to the point where their whole lives and motivations revolve around eating and gathering food, regardless of how poisonous, rotten, or alive it is (see Extreme Omnivore below).
Bizarre Alien Psychology: Beholders have two brains. For some reason they process their data through the emotional part before transferring it to the logical part. Which means, if something is against a beholder's beliefs (which, through genetic memory always amount to Always Chaotic Evil racist monster) it won't ever get far enough to apply to its logic.
Bizarre Alien Reproduction: Many monsters have extremely strange methods of producing the next generation. Aberrations, of course, tend to be the oddest of all, but even more "mundane" beasties can be pretty weird.
In 2nd edition, according to their Ecology article in Dragon, the Lamia Noble, which resembles a human man or woman from the waist up and a giant snake from the waist down, only produces more of its own kind by breeding with humans; mating with each other produces Lamia Commoners, who appear as human women from the waist up and a hermaphrodite lioness, she-goat, doe or antelope from the waist down. And these lamias must, again, mate with humans to produce more of their own kind, as mating with each other produces "sa'irs"; sterile bestial beings that look like someone stuck the horns of a goat on the head of a lion after switching its hindquarters for those of a goat.
Beholders, depending on edition, are either asexual or hermaphroditic, but in either case, reproduction ends with the beholder puking up its uterus so that its offspring can then eat their way free. Once it recovers, it tends to kill and/or eat the ones it thinks look the least like itself.
Illithid reproduction is two-part; first, an adult illithid vomits up or lays (it depends on the sourcebook) a mass of gelatinous eggs in the Elder Brain's pool, which hatch into tadpole-like illithid larvae. Assuming they aren't eaten by others in the pond or the Elder Brain itself before maturing, they are then inserted into the ear of a helpless humanoid, whereupon they consume its brain and physically merge with its spinal column to mutate it into the iconic cthulhumanoid form of the adult illithid.
D&D Sphinxes consist of the Andro- and Gynosphinx (human-headed), the male and female respectively of the species, and two exclusively male parasitic "cousins" in the form of the Crinosphinx (ram-headed) and the Hieracosphinx (eagle-headed). Gynosphinxes only want to mate with Androsphinxes because those are the only partners who will father new gynosphinx daughters, which causes crinosphinxes to try to bribe the gynosphinxes into having sex and the hieracosphinxes to simply resort to rape. At the same time, androsphinxes are prudish intellectuals who dislike sex due to it distracting from "purer" intellectual matters. So, courtship for a gynosphinx requires tracking down one of the reclusive androsphinxes, avoiding being raped by a hieracosphinx or captured by a crinosphinx along the way, and then jumping through hoops to coax the androsphinx into laying off the books long enough to get her pregnant.
In early editions, many monsters had infravision (seeing in infrared), such as bugbears, dragons, dwarves, elves, gnolls, gnomes, goblins, Stout halflings, imps, orcs, quasits and trolls.
Much more rarely, some monsters had ultravision (seeing in ultraviolet), such as bone devils and ice devils.
Later editions simplified these two vision-types to 'darkvision' and 'low-light vision'. Carried to an unfortunate extreme in 3rd Edition, where humans were one of only three species that did not have one of these two vision types.note The other two were halflings and lizardfolk.
Other D&D senses include Blindsense (several forms) and Tremorsense.
Bow and Sword, in Accord: Rangers started off the archetype, but 3E Fighters are probably more effective at it due to their abundance of feats. Certain Rogues and Bards can pull this off too, especially with 4E. Within a broader scope, it's generally a good idea for everybody to have two weapons, one for melee and one for ranged, and that goes for big spellcaster types too — a Wizard lacking a crossbow and fighting in an anti-magic field is utterly useless.
Illithid elder brains, which float in large brine pools in illithid cities. They are made up of the combined brains of old illithids that sacrificed themselves to join it.
The Intellect Devourer is basically a brain running around on four little legs. Its modus operandi is to crack a victim's skull open, remove the brain and then squeeze inside, taking over the body.
Breath Weapon: A very popular technique, and a number of creature variants consist of that creature but with the ability to breathe fire. Not that they don't get a bit creative, as most dragons have a different type of breath weapon, and there are a lot of dragon varieties. Apart from the standard Fire/Ice/Lightning and various varieties of poisonous gas, some of the more unusual ones that have appeared are magnetism, hot sand, thorns, dismissal (they breathe a spell effect that dispels summoned creatures), "antithetical energy" (turns things it touches inside out), shrinking, and spitting a purple "lozenge" that then explodes.
Cat Folk: A number of examples, including the Trope Namer Catfolk, a nomadic Beast Man species reminiscent of lions, found in the Races of the Wild rule book.
Another nomadic leonine Dungeons & Dragons species are called the Wemic. They are centauroid lions. Wemics are excellent hunters and fighters. They do not make settled homes, but generally follow the herds they hunt for food, in the manner of a lion pride.
The rakasta from the Mystara setting are another anthropomorphic cat-people in D&D, the most known subrace resembling domestic cats with very un-domestic personalities. A Dragon Magazinearticle featured a vast array of rakasta subraces, from alley cats to ocelots and lions to smilodons.
Pathfinder has a straight-up "cat race" actually called "Catfolk", and it also has the maftets, a race descended from Sphinxes.
The tabaxi are a race of leopard people who live in tropical jungles. The Forgotten RealmsSpin-Off setting Maztica featured a race of jaguar people also called tabaxi; it explained that the name of the leopard-tabaxi from the Realms was pronounced "ta-bax-ee" while that of the Maztican jaguar-tabaxi was pronounced "ta-bash-ee", but no justification was given to how two different species of cat-people on opposite ends of the world could have the same name.
4th Edition's Player Handbook 2 includes the decidedly feline-looking Razorclaw Shifter, descended from weretigers.
Actually, Shifters were introduced in the Eberron setting. They were brought into basic 4E because they were surprisingly (Or unsurprisingly) popular.
The Tibbit race, which are Small humanoids with cat ears and markings as if their skin were fur; they can also turn into full cats in the manner of a werebeast.
Charm Person: The Trope Namer. There is a spell of that name that does exactly that. It used to work for up to a month per casting, depending on the intelligence of the person who was charmed.
There are improvements, like Mass Charm. And if that's not enough, Forgotten Realms has Virus Charm spreading by touch from the primary target to several secondary targets, thus beguiling people too well guarded to be charmed directly.
The Chessmaster: A frequent villain type. Notable examples include Vecna, who is quite literally the god of this trope, Lolth, who rules an entire race of this trope, and Graz'zt, who manages to be this trope while also being a demon, which is a bit like being a nuclear physicist while constantly high on paint fumes.
4th Edition gives a possible explanation for why Graz'zt is much more sane and more of a chessmaster than most other Demons (besides Dagon). He used to be an Archdevil.
Color-Coded for Your Convenience: Dragons, some Sub-races, Gear, etc. This is a pretty common trope overall in this system. Even the planes of existence have this. Generally, planes with nice alignments have pretty colors; less pleasant planes tend to be black or blood-red. 4th Edition and Eberron decided that made the game too easy, so they did away with it.
Warlocks have powers often derived from evil beings, but can use them for good.
Tieflings even more so: horns, fangs, reddish skin, Glowing Eyes of Doom, and a taste for spikes and red leather (plus some sort of infernal ancestor in the family tree) — yet they're a playable race just like any other racenote The fact that they are random throwbacks that can occur in any child that has some fiendish blood, thus not needing a developed culture/civilization/etc. and the fact that one version has stats that may be worth taking over humans makes them more playable in some ways, and have no fundamental bias towards a particular alignment. To the point where the Tiefling who was Actually Good was a bit of a cliche in the latter days of 3.5. 2E. Planescape had tiefling Rhys — being the high-up of Transcendent Order, she's as close to the embodiment of neutrality as a mortal can get.
The 4E Assassin taps into the Shadow power source, and despite having a legacy of being a back-stabbing dirt-bag, the Assassin welcomes PCs of any alignment. The Shadowfell, from whence they draw power isn't evil per se, just creepy. The Plane Of Shadow, one part of its inspiration from previous editions, is both explicitly not evil and somewhat less creepy. The Plane of Negative Energy, the other half of the Shadowfell's "parentage", less so. Not only is it the source behind all undead in The Multiverse and stated to cause them to be "Always Evil," being there while being alive and devoid of protection means the life gets sucked out of you in a matter of minutes.
Several 3e prestige classes, such as the Malconvoker and Gray Guard, were created specifically around the concept of using dark powers for good. Played semi-straight with fiendbinders, who couldn't be good but could cheerfully be neutral.
The Shadar-Kai are demihumans who changed to their current state after emigrating to the Shadow Plane. In 3E, they were typically evil, but they were The Fair Folk who accidentally disconnected themselves from the natural world and forcibly bound their souls to the Plane of Shadow, meaning that they were slowly fading away into nothing. In 4E, they still have a thing for black leather, spikes, and extreme sensations (pain or pleasure — starting to sound familiar yet?), but they are not inherently evil, and in fact the neutral deity of death is their racial patron.
Pretty much every one of the Heroes of Shadow from D&D Essentials embodies this trope. You can even play as one of the undead — the revenant, one of the races, is someone Back from the Dead to do the will of the Raven Queen, and the vampire, one of the classes, is exactly what it sounds like — a creature of the night, normally one of the most evil creatures in a D&D world, who has managed to free themselves from the control of their sire and retain some form of their humanity, and now seeks to do at least some good in the world.
In all editions there are characters and monsters who can fight while at negative hit points, but it came up more frequently with 3rd's feats and prestige classes. 4th edition gives most Epic Destinies (and thus most level 20+ characters) a means to cheat death daily, either with instant healing, a sudden transformation (like into a platinum dragon or a spell-slinging spirit), or a simple self-resurrection seconds later.
One Epic Destiny actually has a future version of your character appear to protect his past self.
Dig Attack: The Miner is a manta ray-like monster that hunts by burrowing up to within a few inches of the surface and sticking its poisonous barbed spines up through the surface. When a creature steps on the spines they're paralyzed for 20-400 minutes, during which time the miner eats it. It is very difficult to attack the miner unless it's dug out of its hiding place.
Disney Villain Death: 1st Edition module I10 Ravenloft II: The House on Gryphon Hill. At the climax of the adventure the evil Creature and the Alchemist Strahd topple over the edge of a cliff and fall to their deaths below.
Disposable Vagrant: Several adventures and supplements have examples of monsters that use this technique.
Does Not Like Shoes: Quite a few monsters are typically depicted like this, but special mention goes to the kobolds in 3.5E, where the thought of footwear never even occurred to them (see the book "Races of the Dragon" for details.) Note that this doesn't stop a player who is using a kobold in that edition from equipping a pair of shoes or anything similar.
Halflings may or may not be like this, depending on whether or not the DM wants to treat them as Hobbit-expies
Dragon magazine had an article on Hags which stated that they mate with human men, then eat them and place their human-looking daughters with human families until they turn into Hags at puberty.
An online article about the Eberron setting, where Half-Elves are an established race, stated that with humans their progeny had a fifty-fifty chance of being human or Half-Elf. But if a Half-Elf married a full Elf their kids would always be Half-Elves.
Draconic Humanoid: Named after the "humanoid (draconic)" creature type from Fourth Edition ("humanoid [dragonblood]" in earlier editions), which includes Half-Dragons and Dragonborns, the latter of whom gives us the page picture. 3.5E devoted an entire sourcebook to these, Races of the Dragon, giving massive amounts of detail on creatures ranging from the Dragonborn of Bahamut (not the same thing as the 4E dragonborn race) to the humble kobold. It also introduced the dragonwrought kobold, one born with more draconic features than usual (functional wings, for instance), and which under certain interpretations counts as a True Dragon, like Red or Silver Dragons (this matters as there are a number of feats and other options limited to just true dragons).
Dragon Hoard: All dragons have hoards, although their content varies from species to species. This is reflected by the rules giving them a much larger amount of loot.
Dragon Rider: The cover of Basic D&D's Master Player's Book shows a majestic king riding a gold dragon. He holds on by the hair on the back, and has a sword drawn, ready to fly into battle. While that installment of Basic D&D doesn't give explicit rules for a dragon-based mount in the core rulebooks, it does include stronger versions of dragons that would require much more effort to subdue, thus making it only an artwork depiction.
Dragons Are Demonic: Dungeons & Dragons has both good and evil dragons, Color-Coded for Your Convenience. Though both are hoarders of treasure, chromatic (colored) dragons are full-on evil 99% of the time. There are also evil draconic deities, including Tiamat, a goddess of greed who is also a member of the Forgotten Realms' main pantheon. Metallic dragons are usually good-aligned, but fall under Good Is Not Nice.
The dragons have their own pantheon of draconic deities. Two of them, Bahamut and Tiamat, are also worshiped by humans in the Forgotten Realms. Tiamat is the goddess of greed and Always Chaotic Evil chromatic dragons, while her brother Bahamut is god of Always Lawful Good metallic dragons, justice, and strength.
D&D Rules Cyclopedia. In the BECMI (Basic/Expert/Companion/Master/Immortals) D&D system, the dragons of each alignment (Lawful, Neutral and Chaotic) have their own ruler (Diamond, Opal and Pearl respectively), with the Great One (Ruler of All Dragonkind) ruling over them. Each of these ruling dragons is an Immortal (the BECMI equivalent of a deity).
Dreadful Dragonfly: Giant dragonflies appeared as a new monster in module EX2 The Land Beyond the Magic Mirror and later in the Monster Manual 2. They are man-sized, have up to 64 Hit Points, and are very hard to hit in combat. They are known to attack human beings, and can kill a normal human in a single combat round.
Genies. Djinni can change into gaseous form and Marids can change into liquid form.
Elemental Grues. Harginn can change into fire and Varrdig can change into water.
Emo Teen: The Maenads (stop giggling), a race introduced in the Expanded Psionics Handbook. They're pale-skinned, black-haired and were wronged aeons ago by their parents the gods. Their stoic, intense exterior belies their boiling internal rage, which they release by screaming. Also, they sparkle.
Enthralling Siren: Interestingly, the harpy is closest to the original meaning of siren—bird women with a voice that draws victims closer. A siren is humanoid, and her voice charms hostiles.
Even Evil Has Standards: The Drow, for all their depravities, are utterly disgusted by the unthinkably insane Derro, and slaughter them whenever they can. As compared to Illithids and Duergar, canonically trade partners when not at war. Evil dragons usually stick to draconic codes of honor. Also, the Demon Lord Kostchichie is said to be so evil even the other demon lords hate him.
Evil Counterpart: The game makes much use of this trope. Most prominently, the drow are to other elves. Other examples include:
Duergar and derro are evil counterparts of the rest of the dwarves.
Githyanki are the evil counterparts of githzerai (though the githzerai aren't all that nice themselves).
Paladins' evil counterparts anti-paladins, first introduced in Dragon Magazine for 1st Edition. In 3rd Edition, they're a prestige class called blackguards (though Dragon also ran an article offering wildly different "holy" or "unholy" warriors for all nine alignments, including anti-paladins for Chaotic Evil — and a character can be both an anti-paladin and a blackguard at the same time). 4th Edition (and its reduced emphasis on alignment) didn't bother anymore, with paladins being divinely empowered warriors that could serve any god or faith, though the Essentials books went back to the previous mold with two paladin subtypes: cavaliers that had to be good and blackguards that had to be not good.
Basic D&D (of the "Red Box"/Rules Cyclopedia ilk) introduced the paladin and its counterpart together — at high level, a lawful fighter could become a paladin, and a chaotic one could become an avenger. 5th Edition's core rules are set to do something similar, with paladins being lawful, but also having three subtypes roughly across the good-evil axis — cavaliers, wardens, and blackguards.
The Greyhawk gods and half-brothers Heironeous and Hextor (Hextor is the evil counterpart).
The svirfneblin, or deep gnomes, seem designed to make players think they're Evil Counterparts for the friendly surface-dwelling rock gnomes, but they're actually very shy and retiring.
Red and gold dragons, in 3rd Edition especially, they were the strongest of the core chromatic and metallic dragons respectively and viciously opposed to one another. Red and silver dragons more so: same CR, live in the same area, one's a chaotic evil fire dragon, the other is a lawful good ice dragon. The 3rd edition Draconomicon also says they have similar silhouettes from below.
Stronmaus, god of storm giants and good cloud giants is the brother of his evil counterpart Memnor, god of evil cloud giants.
The magic item called the Book of Vile Darkness is the evil counterpart of the magic item called the Book of Exalted Deeds, and the game also has rulebooks with the same names about everything evil and good (respectively) in the game's universe
Pretty much any spell in 3.5 that has the "Good" or "Evil" descriptor has a counterpart with the other one. Often, it even extends to "Law" and "Chaos."
Extreme Omnivore: Hill giants will eat pretty much anything they can get their hands on- plants, animals, humans, dwarves, elves, and even rotten or poisonous foods. They're not immune to such nasty stuff however, and often get sick as a result (but go right back to eating them anyway, since they're not particularly bright).
Eye Beams: Lots of monsters, including classic, like basilisk and medusa and less classic, like beholders and bodaks.
The Fair Folk: Some fey are the happy, helpful little fairies of modern pop culture (atomies, pixies, flitterlings), others are murderous little blighters (redcaps, gremlins, quicklings), and others are the genuinely terrifying godlike beings of ancient lore (Ravenloft's Gwydion, the Wild Hunt, the Primordials).
Fauns and Satyrs: The satyrs, who are a combination of the fauns and satyrs of Greek Mythology. And also there are ibixians, a race of goat-men best known for their... teamwork? Huh?
Fearless Undead: The undead in many editions are immune to all fear spells, and if morale is a factor in a game, the undead are near the top of the heap. The only thing that really scares undead is holy power, such as that unleashed by the Cleric's Turn Undead ability.
1st Edition supplement Deities & Demigods Cyclopedia, Greek Mythology section. When Zeus takes damage and his blood spills on the ground, the blood changes into a powerful random monster that is under his control until it dies or 48 hours elapse, whichever comes first. This can occur each round of combat.
Several versions of the default pantheon include the tale that elves and orcs first sprang forth from the blood of their respective patron deities, who'd fought a vicious duel that scattered it across many worlds.
3.5 has a spell that burns a corpse in a special way so that there is a 50% chance even the most powerful (at least non-epic) resurrection spell won't bring it back.
If a person is killed by a corporeal undead creature, they may become an undead of the same type a certain amount of time later. One way to prevent this from happening is to burn their body to ashes before the deadline.
And of course, trolls, which will eventually regenerate from any damage, even apparent death, unless burned with fire or acid.
Fog Feet: Angels as well as water and air archons in 4th edition.
For the Evulz: Surprisingly rare; even demons and devils frequently have motives one can understand, if not exactly sympathize with. However, does get played straight on occasion — for example, the third edition Monster Manual says hags sometimes appear to "do evil for its own sake".
From Nobody to Nightmare: According to the fluff, fiendwurms, enormous serpentine beings with extremely dangerous magic abilities, are created by applying demonic magic to... an ordinary earthworm. Yes. In the blink of an eye, one can transform common vermin from a gross, if strangely adorable annelid into a train-sized killing machine.
Frog Men: Several species; the blindheim, the grippli, the slaad and the bullywugs.
A number of monsters that are made up entirely of crystals and/or gems.
The crysmal is a creature from the Elemental Plane of Earth that is made up of large crystals. It eats gems such as quartz, beryl (emeralds and aquamarine), corundum (rubies and sapphires) and carbon (jet and diamonds).
In the Forgotten Realms setting, the Red Wizards of Thay created gemstone golems made of diamonds, emeralds and rubies.
Dark Sun Monstrous Compendium Appendix II: Terrors beyond Tyr. The dagorran is a giant frog-like creature with green crystals growing out of the top of its back, under the neck. These crystals are the source of the creature's tracking and psionic abilities. The leader of a pack of dagorran has a larger growth of crystals than the other pack members.
1st Edition AD&D supplement Fiend Folio. The carbuncle is an armadillo-like monster with a large ruby in its forehead. If the ruby is removed it will grow back over a period of several months.
The underground sea-dwelling Starfish Aliens known as Aboleths exhibit this in Dungeons & Dragons (3rd edition, anyway). Each of them inherits every single memory from its parent, resulting in a staggering amount of information being in their head at birth, and allowing two Aboleths to see how they're related based on how far back their memories diverge. What's really creepy is that these memories go back farther than the creation of the world...
Aboleths gain the memories of creatures they eat. And, like the Goa'uld, have genetic memory that reaches back eons. They remember a time when they ruled the world. They are understandably bitter about the current state of affairs.
They in fact can remember a time before gods came along and created the world.
Multi-Armed and Dangerous insectoids Thri-Kreen ("mantis warriors") have racial memory which isn't readily available, but is awakened by some reminders, piece-by-piece. Includes necessary skills like their language (spoken and written), how to make construction material from saliva, typical designs based on this material (like throwing weapon) and other interesting things.
Dragons basically are able to pass along edited instincts through their genes — so yes, if a dragon researches some new spell, its children can learn it automatically. Or, if some evil empire nearly kills the parent (before the eggs are created, obviously), the children will know to avoid that kind of thing without being told. Given that most dragons are probably not great parents this is one possible way they know things like language, that or magic.
Genius Bruiser: Baphomet, one of the myriad Dimension Lords of the Abyss, has a body that is exactly what you would expect from someone who calls himself the "Demon Prince of Beasts"... and has the brain of a Chessmaster, preferring to destroy society from within before attempting to raze it to the ground.
"Basic" D&D in particular had golden dragons as the only "metallic" example — the usual chromatic spread of black, blue, green, red, and white (interestingly not all "evil" respectively "chaotic" — whites and blues were neutral in this edition), plus gold... and that was that. Golden dragons were still the most powerful here, having the best stats and being the only type to get two distinct breath weapons and have guaranteed spellcasting abilities (an explicit 100% chance on what would otherwise be a random roll) on top of that.
The Goomba: Goblins and kobolds typically fill this role, though if their strengths (sneaking and trapmaking, respectively) are played up, they can easily challenge more powerful adventurers.
Gorgeous Gorgon: Many medusae are quite beautiful, but one look into their eyes will turn you to stone.
Heroic Dolphin: Some editions have depicted dolphins as sentient Good-aligned creatures with their own patron goddess.
Hear Me the Money: In the Complete Book of Villains, a 2E supplement, a dragon is presented as an archetypical villain representing greed. When its minions bring it tribute, it listens to the coins being poured out onto its hoard, and immediately detects from the sound that one of them has cheated it.
He Who Must Not Be Named: He Who Was, the god Asmodeus slew to fuel his apotheosis, is a literal case, as he now has no name. The Devils literally erased all knowledge of the god's name from existence, as simply saying it would be enough to resurrect him. At least in the Points of Light setting. In the Forgotten Realms, it's Azuth, and people are quite aware.
Hobbits: How closely halflings have stuck to the traditional Tolkienesque model has varied over the years, generally less so as time goes by. There's also the kender, but they've been different from the start. Eberron gives them velociraptors.
Honest John's Dealership: One of TSR's add-on books for 2nd edition AD&D had an Underdark merchant playable class. As a class perk, this character is not only expected but required to moderately cheat any customers. If the character does a completely honest transaction, underdark NPC's such as Drow assume it's a ruse for something even worse and automatically attack.
Horror Hunger: Libris Mortis: The Book of Undead goes over which undead suffer from this and what it is like.
Horny Devils: Succubi and Incubi, and a few other types of demons qualify as this.
Hufflepuff House: invoked There are three main types of fiends in Dungeons and Dragons. The Chaotic Evil Tanar'ri (demons), the Lawful Evil Baatezu (devils) and the Neutral Evil Yugoloths (daemons). The Tanar'ri were the pillage-and-burn kind of evil and the Baatezu were the scheming-and-making-deals kind of evil. This led to the writers struggling to come up with a way to properly define the Yugoloths. They settled on making Yugoloths masters of conspiracy. Whereas devils focused on individual schemes, Yugoloths focused on vast webs of intrigue that spanned centuries. It never quite took, and Yugoloths tend to be overlooked both by the players and by the developers.
Humans Are Average: A cornerstone of practically every edition. In 1E and 2E, humans had no penalties or bonuses and could be any class with no class limitations. In 3E, humans got a bonus feat at first level, an extra skill point at every level, and an easier time multi-classing because whatever their highest class level was considered their racial class. In the current edition of the game (5E), humans get either +1 to all ability scores or +1 to two ability scores of the players choice, an extra skill proficiency and a free feat. In addition, while elves and dwarves have their own cultures, patron gods and typical alignments, humans can come from all walks of life and all different cultures.
An Ice Person: Racial variants of the cold elemental variety, plus Uldras.
Increasingly Lethal Enemy: The 1st Edition supplement Deities & Demigods Cyclopedia, Greek Mythology chapter. The giant Antaeus grows stronger each round he fights as long as he's touching the ground. He starts with the strength of a hill giant (19) and gains the strength of the next larger giant size each round of combat until he reaches the strength of a titan (25). This allows him to do increasingly more Hit Points of damage when he hits. In addition, each time he grows he regenerates all lost Hit Points of damage and gains an additional 25 Hit Points, making him tougher to kill. So when he starts to fight he has 100 Hit Points and a Strength of 19, doing 2-16 Hit Points of damage per hit. After 7 rounds of combat he has 250 Hit Points and a Strength of 25, doing 8-48 Hit Points per hit.
Inhumanly Beautiful Race: Quite a few species are described as this. The most notable are the nymphs, described as being so beautiful that they can make characters go blind just from seeing them.
Insane Troll Logic: In 4E, this is a Slaad's way of thinking thanks to being made of chaos.
The drow deity Lolth can take the form of a giant spider. She has complete command over all types of spiders and usually has a variety of spiders and spider-like beings attending her.
Module WG7 Castle Greyhawk, level 7 "Queen of the Honeybee Hive". The Big Bad of the level is Aunt Bee. She was originally a human being, but was turned into a giant queen bee by her addiction to royal jelly. She rules over all of the bee (and bee-like) monsters in the level.
Abeils, bee-people in the third edition Monster Manual 2, have a Queen. She's portrayed as pretty much a regular abeil in a fancy dress.
Jobber: Regdar, the iconic 3rd Edition Human Fighter, infamously has pictures in almost every splat book of him getting or having been beat up, including at least two where he's been killed (albeit both show him about to be raised, which is relatively easy in D&D). Monte Cook has claimed it was a Writer Revolt over demands to not only make the iconic Fighter a white human male but to make him prominent in art as well.
Just Eat Him: Anything with the "Swallow Whole" ability. Most notorious example: The Purple Worm. They call him "purple people eater" for a reason...
Kaiju: The tarrasque, as well as larger dragons. Advanced and epic dragons are so large that another size category (Colossal+) was created just for them.
Kill It with Fire: Several monsters, notably trolls, most hydras, and some undead. As a general rule of thumb, if it regenerates and you can't drown it in acid, Kill It with Fire. On the other hand, often subverted in that fire is the most common magical attack form for monsters to be immune or resistant to, and there are also fairly common spells for taking much of its sting away.
Knight in Shining Armor: Paladins in general. The sourcebook Book of Exalted Deeds is designed to help players create characters who fit this trope.
Kryptonite Factor: Trolls can regenerate from any wound except those inflicted with acid or fire. The same for most regenerators. Specific weaknesses like silver and holy water, if any, frequently do this too.
Lazy Dragon: In 1st Edition Advanced D&D, all dragons encountered inside their lairs had a percentage chance of being discovered while asleep. This ranged from a low of 5% for Bahamut the Platinum Dragon to a high of 60% for a white dragon.
While aasimar, the celestial counterpart to tieflings, can have features like glowing eyes, radiant skin, and possibly even angelic wings, they're capable of being evil just like tieflings are capable of being good — and even if they're not, can be a Knight Templar.
Also, in 4E, there's the Radiant keyword, a light based energy from the Astral Sea, which is commonly used by clerics and paladins (regardless of their actual alignment, so even evil clerics have radiant powers). Also, it's a common keyword for Star Pact Invocations, which are gained from the strange eldritch entities from beyond. Somewhat averted in the 4e Dungeon Master's Guide which suggests that divine characters dedicated to evil gods are more likely to do necrotic damage and have divine NPC characters replace the radiant keyword with necrotic. Though it's not advised that PCs be evil in alignment, so for a player to do the same thing requires DM intervention.
Pazuzu has the title of Angel of the Five Winds and is often a patron of good-aligned people. He's also an obyrith, which in 4E means he's partially responsible for the creation of the Abyss, and for his current modus operandi, well...
Literal Split Personality: Bahamut and Tiamat in 4e. According to the Monster Manual, a deity named Io, who created the dragons, went up against a primordial named Erek-Hus. The primordial cut Io in halfin one swing; the left half regenerated into Bahamut and the right into Tiamat, who managed to kill Erek-Hus. Tiamat got Io's hubris and arrogance, and Bahamut got Io's protective nature and his fairness.
Demogorgon is another example. During a battle with a god of justice, Demogorgon was cleaved from skull to collarbone, but the net result that each half of his head reformed into two whole heads, each with its own name and personality. Oh, and they plot against each other.
The Living Wall, which gains power by assimilating nearby corpses.
Second edition had the "Greater Mimic". A larger version of the game's resident Chest Monster which could impersonate rooms or other structures.
First Edition had the "Lurker Above" (just called the Lurker in Advanced D&D), which resembled the ceiling.
Its counterpart, the Lurker (renamed the Trapper when the LA had the "above" dropped) disguised itself as the floor.
The Dread Gazebo is an accidental version of this which came about when a player assumed a "gazebo" the Dungeon Master described was some sort of monster. Fed up with his insistence on attacking it, the DM decided it pounced and killed his character. Read the original story here.
This is also played around with in module S1 Tomb of Horrors, as the fake lich, once destroyed, causes the room and dungeon to appear to collapse. Gary Gygax nearly explicitly encourages Dungeon Masters to be sadistic in describing the collapse and ruthless in enforcing a countdown. To those who leave, Gary suggests the DM ask "was that too hard for you?" or words to that effect, while those who stay are NOT crushed. (One almost wonders if early drafts had instructions on how to get tar and feathers out of clothes and hair.)
Made of Plasticine: Monsters marked as "Minions" in the 4E Monster Manual never have more than 1HP, despite their level (but can't be injured on a "miss".) The intent is to simulate Mooks.
Magic Knight: Many fighter/mage classes (or "Gish", as they're known in fan circles); including the duskblade, hexblade, and swordmage(from 4th Edition). On the divine side, there's the Paladin and certain cleric builds.
Magic Music: Bards, who fight orcs with their magic-infused music.
Mama Bear: Mother dragons and cheetahs and female lizardmen in 1st Edition, and Grey Renders.
Mark of the Supernatural: According to the 3E Races of Faerun, aasimar, humans with a good-aligned extraplanar creature in their ancestry, can range from quite humanlike to obviously supernatural, but even the humanlike ones generally have at least one identifying mark that belies this. An aasimar descended from a solar frequently has vestigial feather patches at the shoulder blades, and one descended from a celestial serving a deity may have a birthmark in the shape of that god's holy symbol.
The Modrons and Inevitables are classic examples, as they're the examplars of Order (Read: Lawful Neutral).
The Warforged are a more recent example. Originally hailing from Eberron, these Magitek androids became incredibly popular — they've even managed to migrate to the base setting of 4E with another Darkhorse, the Shifters.
Liches are undead who were high level spellcasters in life. Many of them are stated to have achieved lichdom in order to avoid dying of old age.
Module OA7 Test of the Samurai. The Big Bad Za-Jikku is so determined to avoid death that he plans to turn the entire planet's atmosphere into a lethal gas that only he can breathe and which will grant him immortality. The fact that this will kill off all other creatures on the planet does not concern him.
Mugging the Monster: Back in editions where metallic dragons were good, one of their favorite tactics was deliberate usage of this trope via shape shifting (something all metallic dragons have upon birth). They shape shift into something weak and defenseless and wait for some evil aligned idiot to take the bait. You get one guess as to how that turns out.
Multiple-Tailed Beast: The demon lord Demogorgon has two tails. The Warped Beasts also have multiple, flail-like tails.
Nigh-Invulnerability: Some of the more powerful monsters are quite difficult to hurt. Special mention goes to the tarrasque, which in 3e has damage reduction only penetrable by epic weapons, massive spell resistance, immunity to just about anything that would weaken it, a shell that either reflects or negates many offensive spells, and, most famously, rapid regeneration that can only be overcome by reducing the thing to -10 hit points and then altering reality to keep it dead; it will regenerate even if disintegrated. The 4th and 5th Edition tarrasque, it should be noted, no longer has the regeneration, and the 5e version has somewhat lower overall stats than the 3e version (for example, an AC of 25 instead of 35, 676 hp instead of 858, +19 to hit instead of +57, and so forth). Of course, it's still a massive powerhouse, and it still retains most of its other defensive abilities, but at least now if you somehow manage to kill it, it stays dead. It's a little less invulnerable and a little more "nigh".
Warforged are explicitly genderless and cannot reproduce. A warforged who decides it's "female" could, if it wanted (and could find a women's Big and Tall shop), dress the part. Or it could decide to not change its appearance in any way and simply insist that its name is "Glenda". Arguing with it is probably not a good idea; they're called warforged for a reason.
No-Sell: Each monster has a cornucopia of various immunities. Some of them, very many. At later levels, it becomes possible to ignore immunity to some extent.
One to Million to One: In 4E, the crystalline beings known as the Shardmind have an ability that lets them separate into individual pieces and reform a short distance away after being successfully attacked.
Early editions had Warhorses which a Paladin could befriend. Each Warhorse/other mount (often they were some kind of Cool Steed) is specifically bonded to a particular Paladin, and won't respond to others without their master's order.
Dragon magazine #149 "Dragon's Bestiary" article. The Kiita is a horse-like creature that chooses its rider based on specific criteria. The rider has to be one of the Good alignments (Lawful Good, Neutral Good, Chaotic Good), more intelligent than the kiita, and a member of the monk, cleric or ranger class (chosen in that order if there is more than one potential rider).
2nd Edition Forgotten RealmsElves of Evermeet supplement. Moon-horses live on the Isle of Evermeet with the elves. A moon-horse decides for itself if it will act as a mount for a particular elf.
Dragon magazine #34 "Choir Practice at the First Church of Lawful Evil (Orthodox): The ramifications of alignment". Several deities fit this trope: Cyrullia, who appears as a beautiful hemaphrodite, Slarsken Obel who appears as a man most of the time but as a women in matriarchal societies, and Demyuritas, who appears as a stunningly beautiful youth who can be either male or female.
1st Edition Dragon Lance Adventures supplement. The section "Gender and the Gods" says that it's not entirely clear which gender each of the deities is because legends speak of them appearing as either gender at different times. For example, Takhisis is said to appear as the female Dark Temptress or the male Dark Warrior.
Role Aids supplement Witches. The Powers worshipped by Faerie witches can appear as either male or female. In their true form they are neither.
invokedDaemons (called "Yugoloths" in 2nd and 3rd edition), Neutral Evil and from Hades, who just want to spread suffering. (Have officially become demons in 4E, although there's implied to be something different about them.)
The Planescape setting has always had many more, including but not limited to the gehreleths, night hags, barghests, rakshasas, baatorians, kytons, baernaloths, hordlings, avari and diakka, all of which have numerous variants themselves. Let's just say there's a whole lot of evil outsiders available in D&D.
Our Dragons Are Different: Holy mackerel, are they ever. With just core rulebooks you've got your basic metallic dragons (usually good-aligned) and chromatic dragons (usually evil). With add-ons it gets fairly ridiculous, to the point where there are entire supplements written solely about dragons and draconic races.
Our Dwarves Are All the Same: Hill and mountain dwarves fit the trope to a "T" (not only are they the same as dwarves in other stories/games/etc., but they are even mechanically identical to each other) but many divergent dwarf subraces exist: the degenerate gully dwarves of Dragonlance, the bald and (gasp) beardless dwarves of Dark Sun, the polar glacier- and jungle-dwelling wild dwarves of the Forgotten Realms, the evil half-human derro of Greyhawk (and their all-dwarf, but equally deformed and evil Dragonlance counterparts, the Theiwar), the equally evil deep dwarves known as duergar, and so on.
Possibly subverted as of 3rd Edition; in most cases, a Human character (or many, many other races) is more effective than the standard elf. Completely in force in First and Second Edition if you refused to play with caps on non-human levels (as most players did). Also quite present in supplemental materials; elves are impossibly good at making various textiles and music, for example. Played straight and averted in 4th edition with the eladrin (in keeping with the Arthurian-mythology-tinged fey origin of their race) and the elves (their wilder, less inscrutable cousins), respectively.
Our Gargoyles Rock: Ambush predators passing off as stone statues until unwary adventurers draw near.
Our Genies Are Different: For four elements, Efreet, Djinn, Marid, and Dao, then their nobles in case you need the same boosted Up to Eleven. Ruler of any genie kind has power second only to respective Elemental Lord. Plus "composite" Jann which are mostly like humans, only more powerful. All genies are extremely self-important and unpredictable; it's a good idea to avoid even the good-aligned ones.
The Dao are like the Efreet, but worse; they're slavers, but have no honor.
Djinn are the most likely to aid player characters since they hate evil, but they're still dangerously fickle.
Marid are the most unpredictable of the genie races. They're fickle, hedonistic, and incredibly egotistical, living only for themselves and caring little for the consequences of their actions.
Our Gnomes Are Weirder: Early on, the gnomish race was hardly more than a strange dwarf/halfling cultural hybrid that could talk to burrowing mammals and had a penchant for illusion magic. Then came Dragonlance and its tinker gnomes, an entire RACE of Bungling Inventors with Overly Long Names, and even non-tinker gnomes became practical joke-loving comic relief (with the exception of the aforementioned svirfneblin). In 4E, though, gnomish insanity has been dialed back and the core race appears to have settled closer to the svirfneblin concept of reclusive underground dwellers with mysterious fey-like powers.
Our Giants Are Bigger: Dungeons & Dragons has a wide variety of giants, including the stereotypically brutish Hill Giants, shy and reclusive Stone Giants, the Fire Giants (who look like gigantic evil dwarves), and the Norse-inspired Frost Giants. Storm Giants lean more toward the Gentle Giant side of the archetype. Also quite literally bigger. Giants range from Large-sized (about twice as tall as an average human) to Colossal-sized (about 16 times larger than a human).
Our Monsters Are Different: Given D&D's long history, there's several books of Monster Manuals plus a lot of add-ons and supplements. They change a little with each new edition note and now also from third-party products due to d20 open license, and it's inevitable that straight examples, subversions, and aversions of every monster trope imaginable has cropped up in at least one sourcebook or magazine article over the last 30+ years.
The rabbit is not just sitting there. The rabbit is part of the monster. So you're looking at an evil tree stump that has a cute bunny on the end of its tentacles so that it can lure people or other animals near it. While I understand the parallel to animals in the real world, I'm still stuck here looking at a googly-eyed tree stump with a rabbit glued to its head. Wow.
Our Ogres Are Hungrier: Simple minded, short-tempered, and always hungry. Ogre Magi also exist, based on the Japanese Oni (and, appropriately enough, are called Japanese Ogres). 4e decided there was no point hiding the truth and removed Ogre Magi in favor of an outright Oni monster category. While there are several types, such as the Night Haunter and Spirit Master, they are all explicitly described as evil creatures with a vaguely ogre-like appearance and invariably some form of shapeshifting or illusion type power they used to deceive humanoids.
Our Vampires Are Different: Vampires in D&D hew pretty closely to tradition, though they vary in temperament from savage brutes to dignified killers◊ depending on the individual. The horror-themed Ravenloft setting, though, introduced numerous variants, such as the elven vampire, which can only survive in the day and is killed by exposure to moonlight!
Our Werebeasts Are Different: Werebeasts are collectively (and inaccurately) lycanthropes. In addition to werewolves, there werebears, werecats, wererats, wearboars, weretigers, dire wereboars (hill giants that turn into dire boars), and jackleweres, just to name a few. The 3.5 edition Monster Manual has rules for the use of any type of animal as template for a werebeast.
Outside Man, Inside Man: Used in 4th edition's alignment system. Good characters prefer to overthrow corrupt governments while Lawful Good characters prefer to change things from within.
Papa Wolf: Dragon and cheetah fathers in 1st Edition and Rudolph van Richten in the Ravenloft setting.
Partial Transformation: Most werecreatures in 1E, 2E and 3E have this ability. The hybrid form is a mixture of human and animal characteristics.
A number of adventures have included guards who can summon reinforcements.
T1 The Village of Hommlet. When one group of guards is encountered, they make a low hooting sound that brings more guards. If they're being defeated they start howling, which summons all of the remaining opponents.
T1-4 The Temple of Elemental Evil. A group of hobgoblins will strike a gong to alert other nearby guards and bring them to attack.
Dragon magazine #132 article "With All the Trappings": a guard in the Grey Griffon Inn can pound on a gong with a mallet and arouse the entire inn.
Dragon magazine #64 article "The Assassins' Run". In the Forgotten Realms campaign setting, the Shadow Thieves use the title obstacle course for training purposes. In one of the rooms, trainees must prevent two guards from striking an alarm gong or they will fail the course.
Puny Humans: In the early editions, humans had dead-average stat spreads and none of the special abilities (night vision, bonus weapon proficiencies, etc.) of every non-human race. Humans were "balanced" by their ability to reach higher character levels than other races, a poorly-thought-out rule which most players ignored (and by so doing, created this Trope), and by their access to character classes that no other race could qualify for. The predictable result: humans were played only for access to those classes.
This becomes averted in later editions by taking a similar concept and giving more immediate and tangible benefits for it. Humans in 3rd and 4th edition reflect their diversity and commitment to their trade by gaining an extra starting feat and skill (and a third at-will attack in 4th), and though they have only one ability score bonus instead of two, they can put that one into any ability they wish. Fifth edition instead allows humans to either take an extra point in EVERYTHING, or a proficiency and an extra feat, plus two abilities. (Although the former is commonly agreed upon to be a Master of None, the extra feat makes the latter really good. In fact it's arguably a Game-Breaker.)
Recycled IN SPACE!: or, more commonly, UNDERWATER! Nearly every land monster has an "aquatic" or "sea" version, which sometimes makes one wonder if the publishers aren't just trying to pad out the page count of the Monster Manuals.
The Xorn and Xaren, two related stone-like creatures that eat minerals and metals.
The Crysmal, a hexapedal crystalline creature about three feet high. They eat crystals and are preyed upon by Xorn.
Earth elementals were large humanoids made of stone and rock.
The 1st Edition supplement Manual of the Planes populated the Elemental Plane of Earth with creatures that appeared identical to Prime Material Plane creatures (such as bears, jackals and pegasi) but were made of rock and stone.
Creatures from the Quasi-Elemental Plane of Mineral:
Dragon magazine #174. The Dragon's Bestiary had four creatures made of crystal: Glomus, Shard, Trilling Crysmal and Crystalle the Ruler of the Plane of Mineral and Prince of Mineral Quasi-Elementals
The Bowler, which looks like a small-to-medium sized boulder. It rolls over other creatures and crushes them, then eats them.
The Galeb Duhr was a large boulder-like creature which had arms and legs and could use earth-related spells.
Stone Trolls (Dragon magazine #199) had rocky skin that they got from eating rocks, stones and gems.
Construct (created) monsters
The Caryatid Column, which animated and attacked intruders.
Gargoyles. In 2nd Edition they were magically animated sculptures. Dragon magazine #223 had four variant stone gargoyles.
The Stone Guardian, a golem-like creature used to protect specific areas.
Gem Vars in Dragon magazine #56. They were small humanoids made out of diamond or ruby.
Basic D&D had Living Statues, one of which was the Rock version. It had an outer crust of stone and was filled with magma (lava), which it could squirt out of its fingers.
2nd Edition had the three gemstone golems (Diamond, Emerald, and Ruby).
Sacred Scripture: The vast majority of the rather large number of gods have a holy book attached to their faith.
Sanity Has Advantages: Beholders are incredibly destructive and would be a terrifying threat to the world if they weren't so batshit crazy. They can disintegrate matter at will, control minds, kill with a glance and nullify any magic, but they're also all individually convinced that they and they alone are created in the true image of their goddess, and any beholders that look slightly different must be destroyed, even their own offspring. They can't even come together to form a coherent society because they all hate each other so much.
Satanic Archetype: Although for obvious reasons post-Gygax TSR and later Wizards of the Coast were extremely wary of allowing anything that could be even remotely used to put the charges of Satan-worship at their doorstep, there have been a few uses of Satan-like characters in D&D:
Asmodeus, Beelzebub (under the alias "Baalzebul"), Pazuzu and other fixtures in Christian demonolgy are used as villains often in the various D&D worlds, particularly in 1st Edition and 3rd Edition (for most of 2nd, devils and demons were banned or renamed).
Satan himself was statted up in a fan-penned article in an early issue of The Dragon, long before the "D&D=Satanism" panic took off. As a injoke, Satan had exactly 333 hit points.
At this point, Asmodeus, the ruler of the Nine Hells, has become the single most direct Satan analogue in D&D, to the point of being Old Scratch in all but name.
Basic game. Sabreclaw monsters are created in "wings" of 2d10 members. Each sabreclaw contributes 25 Hit Points to a pool shared by the entire wing. For example, a wing of 10 sabreclaws would have a total of 250 Hit Points. Any damage inflicted on a sabreclaw is divided up among all of the members of its wing. Once an amount of damage equal to the wing's pool is inflicted on it, all of the sabreclaws in the wing die.
The third edition creates a system for general swarms. The swarm remains at full strength until its hit points is reduced to zero, at which point it is either destroyed or scattered (depending on damage type.)
The Sleepless: In 3e, elves do not need to sleep, instead opting for a four hours-minimum period of meditation (dubbed "trance").
Silicon-Based Life: Dark Sun Crystal spiders, Planescape creatures native or related to the Elemental Plane of Earth and Quasielemental Plane of Mineral, the Shardmind from 4th Edition and the Sandling and Storoper in the 1st Edition Monster Manual II.
Early editions had a variety of giant spiders, all of whom appeared in packs. The maximum number varied by type, including Huge spiders (up to 12) and Large (up to 20).
Literal swarms of spiders (as well as other insects and small animals) are often encountered — they occupy a ten foot square area, and do damage by being in the same square as their prey. Often they are not easily damaged by conventional means, making them the bane of many a low-level party.
Spike Shooter: Plenty of monsters can fire spikes, quills, spines and needles at their opponents/victims.
Squishy Wizard: D&D is one of the pioneers of this, though not the original by far.
Stalker Without a Crush: In the 1st Edition Oriental Adventures (1985) supplement, if an application to study with a martial arts master fails, the prospective student may begin courting the master, trying to gain his favor (e.g. by giving a small gift or offering). The hopeful student may continue until either they are accepted or they offend the master.
Threatening Shark: Quoth the Dungeonscape book: "When a dungeon builder needs a deterrent, the only thing better than a giant pit of acid is a giant pit of acid with a shark in it."
Tiered by Name: There's various templates that can be applied to a single creature to modify its stats (size, ancestry, and other traits), which are then reflected in its name. Usually a good indicator of a Mary Sue / Min Maxer if applied to a PC.
The Topic of Cancer: From The Book of Vile Darkness: a Plague MasterPrestige Class "cancer mage" gets a sentient tumor as a familiar: the entire idea of the Cancer Mage is that cancer is something disgusting, creepy and, in this case, actually, cosmically evil.
Touch Telepathy: The adventure OA6 Ronin Challenge. In "Episode 2: Kera Valley" the PCs discover the Diuku, red baboons with legs like a giant frog. A Diuku communicates telepathically by touching its head to the head of another creature and thinking two word sentences like "Friend now" and "Share food".
Treants: Treants have been present as Good-aligned plant creatures since the early days of the game. They were openly named Ents in the first editions of the game, but the name was later changed to treants for copyright reasons.
While treants are normally benevolent, those living in the Demiplane of Dread are vicious and aggressive — there's something in the Demiplane that turns all plant-creatures that grow there into homicidal killers, even if they'd otherwise be good guys, and the treants are no exception.
Forgotten Realms: The forest of Cormanthor is home to a large population of treants, which resemble different trees — such as birch, willow or oak — depending on which part of the forest they come from. They are also noted to live in symbiosis with other forest creatures, such as grubs that feed on a kind of mold that infests treants, toxic fungi that grow around their legs and ward off gnawing rodents and bats that nest in their branches and eat parasitic insects.
The Sandstorm supplement, which deals with adventuring in deserts and wastelands, introduces a variety of treant knows as the saguaro sentinel, which resembles a huge, humanoid saguaro cactus. It's True Neutral rather than Neutral Good, but it guards and protects forests same as other treants — it just does so for cactus forests instead. It also has the added bonus that, since it's covered in sharp thorns, it's also effectively immune to melee attacks.
Unequal Rites: Possible Trope Codifier. Many Wizards are jealous of Sorcerers, as they have to endure years of unwavering discipline and intense study to achieve the same arcane power as a Sorcerer who might jjust have an innate talent for magic that they may or may not even care to control. Both look down on Warlocks, who often gain their magic powers from very powerful and often malicious entities, even though good Warlocks do exist. Necromancers, however, are almost never treated sympathetically — good-aligned magic-using characters will be reluctant to use necromancy even to perform good deads, and will look down on those who do.
In 2nd edition AD&D, products of elf/human interbreeding with claim to 50% or more elven blood always had the characteristics of half-elves, while those with less than 50% elvish ancestry were functionally human.
The various types of Planetouched are individuals that are mostly a mundane species (human, elf, orc, etc.) but have some blood from an ancestor at least one generation removed who was of extraplanar origin.
Tieflings are mostly human, but have a fiend somewhere in their ancestry. Aasimar are the celestial equivalent of Tieflings. Genasi have an elemental grandparent instead.
Fey'ri are majority-elf, minority-demon crosses.
Tanarukks are Orcs with fiendish blood.
In some versions of fluff, sorcerers get their inherent magical powers from a distant ancestor (usually draconic).
The 3.5E supplement Planar Handbook introduced heritage feats that allowed characters to have a tinge of extraplanar blood, granting various bonuses.
Unreliable Narrator: Many examples, but perhaps the first to come to mind; why do Kobolds hate Gnomes so much? In the 3e Draconomicon, the justification they give is a story in which the Gnomish god, Garl Glittergold, collapsed the dwelling of the Kobold god Kurtulmak and crushed the first kobold city in envy of how kobolds had become master miners, smiths and builders whilst gnomes had achieved nothing beyond playing silly pranks and telling stupid jokes. In the "Ecology of the Kobold" article, instead, Garl merely prematurely set off a trap that Kurtulmak had been working on to use on the other gods; and even there, there's two versions — kobolds claim that he did it because he was jealous of Kurtulmak's superior skill at setting up a prank, whilst gnomes claim he just did it to see if it'd actually work.
Vampiric Draining: The Cerebral Parasite and Brain Mole, vampires and wights, Vampiric Touch spell, the 3rd Editions Death Knell spell, etc.
Variable-Length Chain: Kytons, also known as Chain Devils, can increase a chains length by 15 feet as part of a super natural ability.
Fueled by hatred and a need for vengeance, a revenant rises from the grave to hunt and kill its murderer. Devoid of any compassion, emotion, or logic, a revenant has but one purpose, and cannot rest until it has found vengeance.
The first villain presented in Exemplars of Evil is Zargath Human-Bane, an orc who seeks to create a powerful and proud orc nation... and commit genocide against humans, elves, and dwarves.
Elder Evils ends its list of world-endingEldritch Abominations with Zargon the Returner, father of the ancient Baatorians who ruled Hell before the baatezu devils did. He forced the human civilization of Cynidicea to kill each other and offer him human sacrifice. In the ancient past, Zargon killed both a Barbarian Hero who had been specifically blessed by the gods to kill Zargon, and then various gods themselves when they tried to avenge their champion. Asmodeus, being non-divine and therefore lacking the weaknesses of the gods, defeated Zargon in the process of taking over Hell for the baatezu and again when Zargon returned from death centuries later. He couldn't destroy Zargon's horn either time, and Zargon could eventually regenerate as long as his horn was still intact, so Asmodeus sealed Zargon's body in stone with only the horn sticking out and buried Zargon and the Cynidiceans alive in their tunnels under the earth. The campaign outlined in Zargon's chapter of Elder Evils outlines the process of his eventual release by cultists and the players' quest to permanently(?) defeat him.
Voice Changeling: The monster known as the leucrotta can mimic the voice of any creature it has heard in order to lure its prey into an ambush.
Voluntary Shapeshifting: Changelings are able to shapeshift into other humanoids, while Druids can adopt a number of animal forms.
Also the backstory behind the Dragonborn race in 3.5E. To show their devotion to the Platinum Dragon/Bahamut, some people voluntarily entered into a ritual and took on draconic traits. This was later changed to simply being another race in 4E.
Will-o'-the-Wisp: The Will-O-Wisp is a monster that haunts dangerous and deserted places like catacombs, swamps and bogs with traps that can kill the unwary (Pit Traps, Quicksand Sucks, etc.). When a victim is killed by one of these hazards the Will-O-Wisp feeds on their Life Energy.
Woobie, Destroyer of Worlds: The Fallen Angel Avamerin from Elder Evils. He served the gods faithfully and dutifully for literally millions of years, only for the gods he dedicated his life to to casually toss him aside like garbage after he made one mistake. The feelings of rage and betrayal ultimately led to him becoming the willing mortal host for the dark god Serthos.
Zombify the Living: The Son of Kyussappearances 1E Fiend Folio, 2E Monstrous Compendium Greyhawk Appendix, 3E Living Greyhawk Journal #1 is an animated corpse that has fat green worms crawling in and out of its skull orifices. Once per round a worm will jump onto an opponent in melee combat with the Son and try to infect the victim. If it succeeds, the target will immediately become a Son of Kyuss.