D&D Monsters: Couatls

The couatl has its basis in Mesoamerican mythology, although it's arguable how much it resembles the original. The name is apparently inspired by that of the Aztec god Quetzalcoatl the "feathered serpent". In reality, coatl simply means "snake" so it doesn't refer to any specific mythic creature taken on its own. Having said that, snakes were important to the Mesoamerican people, and associated with many of their gods. In particular, several of them were feathered, magically merging the features of a flying creature with one that crawls along the ground. They seem to have been regarded as divine beings, which fits with the original 1E description of their habitat and relationship with humans.

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1E

As originally seen, the couatl has a head and body that closely resemble those of a regular snake. While the presence of an extra set of teeth between the fangs is unusual, in other respects, the arrangement of the teeth resembles those of snakes such as cobras, kraits, and mambas, rather than say, rattlesnakes or boomslangs. While the head is at least partially covered with scales like that of a normal snake, the couatl's body is feathered, with longer feathers along the back and what appear to be softer, downy, ones on the underside. 

D&D Monsters: Beholders

The beholder is, perhaps, the single most iconic monster in D&D - the one creature that, more than any other, says "this is D&D" rather than some other fantasy setting. That's largely due to its unique appearance, which is quite unlike anything that exists in myth or legend or, indeed, in prior fiction. It was created, not by Gygax, but by one of the players in his original campaign, first appearing in a supplement for 0E, and has successfully stood the test of time ever since.

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1E

As originally shown, the beholder is a spherical creature covered in irregular, flat, "chitinous" plates. A wide mouth occupies much of the lower half and is lined with small pointed teeth similar to those of reptiles. There is a single large lidless eye above the mouth and ten much smaller eyestalks arranged in a ring around the upper surface. The latter are protected by segmented chitin and have also have lidless eyes, similar to those of a snail or slug, although the eyestalks presumably aren't retractable. Severing the eyestalks doesn't otherwise hurt the creature, which suggests that it's able to shut off the blood supply to a severed stalk - or perhaps that it doesn't have blood. There are no other visible features, such as nostrils.