D&D Monsters: Giant and Phase Spiders

The first edition of AD&D includes a number of giant invertebrates, mostly insects. The idea behind these may be influenced by the "big bug" films of the post-WWII era, starting with Them! in 1954, and they provide the possibility of exotic-looking monsters that nonetheless have to look no further than the natural world for inspiration. 

Once we leave insects and look at spiders, there is an even more obvious inspiration: Shelob in Lord of the Rings. While she, and her relatives in other Tolkien works, were likely the primary inspiration behind the original giant spiders of D&D, there are plenty of other fictional examples. The cinematic classic The Giant Spider Invasion is one such (yes, I've actually seen it; there's 80+ minutes of my life I'm not getting back...) and, from more modern fiction, there's Aragog in the Harry Potter books. Giant spiders aren't found in European myth, but there are the Tsuchi-gumo of Japan, which are usually described as spiders (but sometimes crickets) and, at a stretch, Anansi the spider-god of West Africa.

D&D Monsters: Unicorns

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Unicorns are probably one of the best-known mythological creatures of medieval Europe, perhaps on a par even with dragons. Their origins are likely earlier than this, with the general concept of "one-horned magical beast" going back to at least the Ancient Greeks and having counterparts in other cultures, too. Some of these were aggressively dangerous, but it's the medieval version, appearing in religious bestiaries as an object lesson in virtuous purity that we're most familiar with and that forms the basis of the D&D creature. 

So iconic is the appearance of the unicorn that it's one of the few creatures in D&D that looks pretty much the same in every edition, regardless of artist, and that closely follows its traditional form. Early medieval unicorns weren't necessarily white, and some looked more like a goat than a horse, but such variations have long since vanished from the public mind. Pretty much everyone knows what a unicorn is, and what it looks like.

D&D Monsters: Doppelgangers

Doppelgangers are one of those races that, were they really to exist, would surely have a significant effect on the world and be much on the minds of the general public. That they aren't in the standard D&D universes is largely thanks to the great number of other sentient races that exist, many of which are even more threatening.

The word "doppelganger", which means something like "double-walker" in German, only dates back to the late 18th century. However, the general concept of a spooky double of a person is a common one in mythology, folk tales, and just plain ghost stories going back for thousands of years. It's also a common theme in more recent fiction, with perhaps the alien in the 1938 novella Who Goes There? being a particular inspiration for the D&D version. (The story was later remade as a film titled The Thing, although the black-and-white version Gygax would have been familiar with at the time features quite a different sort of monster).

D&D Monsters: Medusae

While the exact details of the myth evolved over time, in the best-known version of the Greek original, Medusa was one of three monstrous sisters known as the gorgons. As the only one of the three who was mortal, she was eventually killed and her severed head used as a magical charm. As a consequence, she is far better known than her sisters, who have no independent myths. In D&D, for some reason, the word "gorgon" is used for an entirely different type of creature, and "medusa" becomes the generic term for the type of being, rather than a specific individual. This has since caught on in other fictional contexts.

(I am going to use the plural "medusae" here, because that was the way I learned it as a zoologist, so I find it more natural than the form usually used in fantasy games).

D&D Monsters: Dryads

Looking again at D&D creatures derived from Greek myth, we come to the dryad. In the original myths, dryads are a type of nymph, or female nature spirit, associated with trees. Originally, the tree specifically had to be an oak tree, but later on the term seems to have been used more broadly for woodland spirits in general. The term "hamadryad" was used for a dryad so closely bonded to her tree ("hama" means "together") that she would die if it did, but others seemingly had no such vulnerability. So far as one can tell, they were supposed to look like regular human women.

Dryads have occasionally appeared in works of fantasy fiction, most notably in C.S. Lewis' Narnia books, which maintains the distinction from hamadryads. In D&D, dryads and nymphs are different kinds of being, albeit with a number of similarities, but the latter did not make it into the 5E Monster Manual, something that's currently one of my criteria for including something in this series. In the first few editions they are, however, said to be "tree sprites"... but it's not clear what this means, since a sprite is yet another distinct creature in D&D, and one that resembles dryads rather less than nymphs do.

D&D Monsters: Zombies and Skeletons

As I have noted previously, undead in D&D represent at least three different broad categories of being: mindless corporeal, sentient corporeal, and noncorporeal undead. Zombies and skeletons fall into the first of these three categories, distinguished by the fact that they have no will of their own and are effectively automata under the control of their creator. They are also the weakest form of undead, a problem only for low-level characters.

It's well-known that zombies have their origin in Haitian legends, perhaps influenced by older African legends, but probably having more to do with the experience of slavery. In these legends, a recently-dead corpse is re-animated by an evil sorcerer, which it then serves as a mindless slave lacking all free will. The modern conception of the zombie, however, originates with George Romero's 1968 film Night of the Living Dead. Ironically, that film never actually uses the word "zombie", but it has become widely used since to refer to a selection of similar beings in films and other media.

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.