D&D Monsters: Satyrs

Like centaurs, minotaurs, and harpies, satyrs have their origins in Greek myth. The original versions are wild nature spirits and, in particular, representations of unbridled male sexuality... this survives today in the psychological term 'satyriasis' for uncontrollable sexual desire in men. As with centaurs, this part-bestial nature is represented in their physicality, which combines various animal-like features with an otherwise human, masculine, body.

In later times, the Romans conflated satyrs with fauns, nature spirits from their own mythology that were part-goat part-man. This is quite different from how early Greek art shows them as looking, but it has become essentially universal in western lore since. Nonetheless, since "fauns" lack the rampant sexuality of satyrs, it was their name that C.S. Lewis used in the Narnia books, and his fauns were far more pleasant than Greek satyrs were said to be. Otherwise, "satyr" has generally been the more common term in fantasy, and that's the term that D&D uses.

D&D Monsters: Minotaurs

Minotaurs are another creature with their origin in Greek myth. In the original, there was, of course, only one Minotaur, trapped in a labyrinth and slain by Theseus. Contemporary Greek illustrations show a male human with the head and tail of a bull, more obviously a hybrid than the D&D version, but at least broadly similar. However, some of the myths were vague as to exactly what the Minotaur was supposed to look like, and in the Middle Ages an alternative with a centaur-like form, albeit often with horns on the human-like head, started showing up in art. The latter form had some popularity (it was the first illustration of it I saw as a child) but has declined in modern depictions.

Given that crawling around in subterranean labyrinths is part of the point of the original D&D, it's unsurprising that the Minotaur would be included in the game. Here, of course, it becomes a race of beings and thereby loses its capital letter. (As an aside, both "mine-otaur" and "minn-otaur" are legitimate pronunciations in UK English, although the latter seems to be preferred in the US. In Ancient Greek, it was apparently "meen-otaur", so, hey...).

D&D Monsters: Wights

The word "wight", as D&D books are always eager to remind us, originally just meant "person". It was common enough in this sense in medieval English, and into at least the seventeenth century (Shakespeare uses it, for one). After that, it becomes somewhat old-fashioned, and it's unlikely that anyone much has used the word in this sense for over a hundred years, at least outside of poetry. In 1869, however, a translator used the term "barrow-wight" (literally "tomb-person") to describe a form of undead from Norse legends. This term, of course, was later borrowed by Tolkien in Lord of the Rings. Gygax abbreviated it back to "wight" again for D&D, while retaining the "undead" meaning.

It's likely Gygax's coinage that influenced George R.R. Martin when he chose the word to describe his own, more zombie-like, beings. However, in his universe, it's the White Walkers that most closely resemble D&D's wights, although there are a number of clear differences, not least in terms of how they are created.

D&D Monsters: Harpies

Harpies are one of a number of D&D monsters that owe their origins to Greek myth. However, the story is not quite as simple as that, since they actually combine two different Greek monsters into a single being: the harpies themselves, and sirens. Both were said to be creatures that were part bird, part woman, but beyond that, there is little similarity between the two in the original sources.

Although very early descriptions of mythic harpies portray them as beautiful, the great majority show them as monstrous. As is often the case, there isn't complete consistency in the descriptions of which parts are avian and which parts humanoid, although something at least resembling the D&D form is the most common. Sirens were even more variable, and some early Greek artwork shows male examples as well as females. In essence, though, it is really only the signature attack - the siren call - that copies over to the D&D 'harpy', which in other respects, is more closely based on its namesake.

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.