D&D Monsters: Unicorns

Brief Plug: I have a book describing a fantasy city out on DriveThru RPG, including over 70 locations that can easily be imported into other settings. If you might find such a thing useful check it out at its page on DriveThru - you can use the "preview" function to see a list of the included locations.

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).