D&D Monsters: Ghouls

1E

The term "undead", as used in D&D actually refers to (at least) three different categories of being. First, there are the mindless undead, such as skeletons and zombies, which are effectively automata that happen to be made from corpses, rather than from inanimate matter. Then there are what we might term the "wilful corporeal" undead, where some kind of intelligence animates the physical body of the deceased, and finally the incorporeal undead, which are a different kind of entity altogether. Ghouls belong to the second of these categories, although they are unusual in the degree of physical transformation that they apparently go through.

Ghouls are originally a creature of Arabic folklore, in which they are a kind of demon (as in the name of the comic-book character Ra'as al-Ghul) that lives in the desert and lures people to their doom in order to kill them. In the eighteenth century, this was introduced to Europeans by Antoine Gaillard, who added the additional detail that they live in graveyards and eat the dead buried there.  This has remained the standard version ever since, although with significant variation.

D&D Monsters: Lycanthropes

In the real world, "lycanthrope" is just another word for "werewolf"; the word literally means "wolf-man" in Ancient Greek. In D&D, and many other RPGs, however, it's meaning has been expanded to refer to a general class of shapeshifters, of which werewolves are merely the most common example. While others have appeared in supplements from time to time, there are four core types other than werewolves. All of these operate in the same general way, in terms of how the moon, silver, and so on, affect them, and how they pass on the condition, so that what I've already said about werewolves typically applies to all of them, too.

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1E

Wererats

Wererats as such appear to be an invention of D&D, although some other legendary creatures (such as vampires) or evil sorcerers have often been said to be able to transform themselves into rats or similar vermin. Non-shapeshifting "rat-men" do predate D&D, but even they are a 20th century invention, appearing in stories by, among others, H.P. Lovecraft and Fritz Leiber. These are most likely the source of Gygax's inspiration.

Wererats are the lowest rung on the stepwise progression of 1E lycanthropes, although they already have the hit dice of a bugbear, the second highest creature in the 'evil humanoid' progression. Unlike the other lycanthropes they can, right from the beginning, transform into both rat and rat-man forms; the latter, unusually, retain human feet and regularly wield weapons. They live in communities of a dozen or so individuals and, according to 2E normally only breed with humans; the condition is passed on to the child only if the mother is the wererat (which makes sense for a bloodborne pathogen). 

D&D Monsters: Werewolves

Werewolves are a longstanding feature of European folklore, with references to men who can turn into wolves dating back to at least the Ancient Greeks, although the belief does not seem to have become widespread before the Middle Ages. For most of this time, however, werewolves seem to have been thought of as evil sorcerers or (in pre-Christian times) one-off individuals cursed by the gods. This "evil sorcerer" version most closely matches the sort of werewolf seen in the works of Tolkien and C.S. Lewis and is quite different from the D&D version.

That instead borrows from horror fiction, most notably the Universal Pictures Wolf Man films of the 1940s. Many of the tropes we associate with werewolves today were made popular by those films, and, in fact, often don't date back much further than the 19th century. In these films, however, as in some more modern examples such as An American Werewolf in London and Harry Potter, werewolves are portrayed as (mostly) tragic individuals, while in Twilight they seem relatively benign. D&D, like many other RPGs, makes them definitively evil although the potential for tragedy is still implicit in their ability to pass on the condition to others.

D&D Monsters: Trolls

Unlike the ogre, which seems to have a medieval origin, trolls have a pedigree that stretches back into ancient myth. Specifically, they are Scandinavian, where they are nocturnal humanoids, often living out in the wilds and at least suspicious of, if not outright hostile to, humans. Tolkien used them in the way that D&D later went on to use ogres, but retaining the mythic feature of them turning to stone in sunlight.

Many other fantasy works have used trolls in the "large, strong and stupid" role of D&D ogres, with the stone-based giants of Pratchett's Discworld and the grey-skinned mountain trolls of Harry Potter being particularly original or well-known examples. The troll of D&D, however, has no real resemblance to these, or to the mythic creature; Gygax instead stated that his inspiration was the 1961 novel Three Hearts and Three Lions by Poul Anderson, which features a regenerating troll, along with a number of other tropes he adopted for the game.

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1E

Trolls are initially shown as cadaverous humanoids with a green or grey warty skin, exaggerated facial features, including a long pointed nose, and writhing tendrils in place of hair. The eyes are large black wells beneath a pronounced brow ridge. Trolls possess no visible genitalia, and so presumably do not reproduce in the human fashion, and have only four digits on their hands and feet - even then one of the toes appears vestigial.

Physically, they are powerful creatures, able to deliver a rapid series of blows, each more powerful than a typical sword-strike, and they have a rubbery hide that's thicker and more resilient than rhino-skin. Even without their regenerative powers, they are about halfway between ogres and the smallest of true giants in their ability to soak up damage - although some of this could be due to simple combat prowess, or a lack of truly vital organs. They also have acute senses, particularly smell (which would explain the large nose).

D&D Monsters: Ogres

Ogres are a common feature in folklore, in which they are typically powerful and brutish humanoids with a taste for human flesh. The word does not seem to go back any further than the Middle Ages, although the basic concept is, of course, much older, and similar beings exist in a variety of ancient myths. Inevitably, they have been widely used in fantasy literature, although they don't appear in Tolkien, since he adapted the same word-root into his "orc".

The D&D concept of the ogre, however, seems to owe a lot to Tolkien's trolls, and as a big, stupid, foe with no particular frills beyond sheer strength, they seem a popular opponent to throw at low-level characters.

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1E

As described in 1E, ogres are powerful humanoids, closer in size to the smallest of giants than to humans. The image shows enlarged ears and a flattened face with a sloping forehead. Oddly, their feet bend in the middle, and there is a spur (possibly one of the toes) pointing backwards from the heel. In other respects, however, their build is essentially human-like. Their skin is said to vary from yellow to very dark brown, and to have numerous warty bumps that aren't apparent in the illustration.

D&D Monsters: Bugbears

While in modern English, the term "bugbear" is really only used to mean a source of irritation or a recurring problem, it did originally refer to a kind of supernatural being. The first half of the word likely has the same origin as the word "bogeyman", and implies a sort of evil spirit. Whether or not the second half was originally meant to indicate that the spirit in question looked bear-like is less apparent, but it was certainly used as such in the Late Middle Ages. By that time, bugbears, like the bogeyman, seem to have been used more as something to frighten children into obedience (such as, say, not wandering off into the forest) than as anything seriously believed in by adults.

The D&D conception of the bugbear is initially an exceptionally large and hairy goblin. Its function in the original editions is as a means of extending the list of five "evil tribal humanoid" races one further step, and thus providing a continuing challenge once characters had reached 3rd level or so. As with those other races, they have become more detailed and varied in use since.

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1E

In 1st edition, the five tribal races of evil humanoid form a distinct game mechanical hierarchy, running from kobolds to gnolls. Bugbears are essentially the same idea continued one step beyond the gnolls. In fact, almost the only difference between the two races in terms of game statistics is that the bugbear has one extra hit die, and thus a 5% higher chance of landing a blow in combat and (on average) 5 extra hit points.

D&D Monsters: Sahuagin

While mermen have obvious mythological antecedents, the idea of bipedal fish-men such as the sahuagin is a more modern one. This is not to say that occasional pictures resembling such things don't appear in the odd medieval manuscript, or whatever (the Polish bishop-fish is one such example), but there's rarely much detail associated with them beyond the image. Instead, the concept of the sahuagin is more likely to have been influenced by H.P. Lovecraft's Deep Ones and/or the eponymous monster in the 1954 film Creature from the Black Lagoon.

While there are other generally similar-looking fish-men in the D&D canon, the sahuagin have the distinction of being the only one to appear in both the 1st and 5th edition versions of the Monster Manual, so I'll use them as a proxy for the others here. Of note is the fact that, unlike merfolk, they are regarded as "evil" and therefore make suitable monsters, creeping ashore to menace seaside fishing villages or attacking boats.

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1E

As seen in 1E, sahuagin are basically humanoid, but covered in a scaly dark green hide as tough as chain armour. The image shows a somewhat ape-like face with heavy brow-ridges and a spined and webbed crest running from the top of the head down to at least the shoulders and likely further than that. Similarly shaped fins are seen running down the arms and the back of the legs, and there are visible gill-slits on the neck. Physically, they are tougher than typical humans and more intelligent with it.