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.