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).