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).