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).