D&D Monsters: Nagas

Nagas originate in Hindu mythology, in which they are magical beings that can take on various snake-like forms. Some appear as literal snakes and, indeed, the scientific name of the main cobra genus is Naja, based on the Sanskrit word for 'cobra'. Similarly, it's no coincidence that the feminine form of this word is "nagini" - although the exact English spelling can vary.

Often capable of shape-shifting, mythic nagas can take on fully human or partially humanoid form, with the latter more usually resembling the yuan-ti and mariliths of D&D than the shape seen in the game (although this is not unknown). Nagas are generally said to be righteous, if not exactly benevolent, and are often set to guard the treasures of the gods, hence at least the 'guardian nagas' of D&D.

D&D Monsters: Sphinxes

The sphinx is a creature of Greek myth, taking the form of a winged lion with a human face. In the original myths, she is, like the minotaur and many others, a unique creature, and appears most famously in the story of Oedipus. 

Confusingly, however, the term was retrospectively employed (possibly by the Greeks themselves) to also refer to the wingless lion-bodied statues of Ancient Egypt. These aren't the same thing as the Greek monster, and it isn't known what the Egyptians actually called them. They were usually, but not always, male and, judging from the statuary, were probably perceived as good and noble beings, unlike the much more hostile Greek creation. 

It seems to be the case that, in D&D, the male or "androsphinx" was based largely on the Egyptian creature (albeit with the addition of wings), while the female or "gynosphinx" has closer links with the Greek version.