D&D Monsters: Griffons and Hippogriffs

The mythic origins of griffons lie in Ancient Greece, if not earlier - a combination of the king of the beasts and the king of the birds. The specific form of that combination was largely cemented by the late Middle Ages into the one we are currently familiar with, and griffons ('griffin' and 'gryphon' are equally valid spellings) are common features in modern fantasy, especially in RPGs. 

The hippogriff, however, is much more modern; it first appears in a poetic work of fiction written in 1516, where it is described as the offspring of a male griffon and a female horse (which, since griffons were thought to kill and eat horses on sight was intended as something miraculous). Although the existence of one may well imply the other, it's possibly more common than the griffon itself in fantasy fiction, with the most famous modern example being that in the Harry Potter books.

D&D Monsters: Mummies

The concept of the mummy as an undead monster is really a 20th-century invention, beginning with the 1932 horror movie The Mummy. While mummies and immortal Ancient Egyptians had featured in earlier fiction, they were not really in the form we're familiar with now, or that formed the basis for the D&D monster, and still don't date back any further than the mid-19th century. Since their 1932 debut, however, they have appeared in a number of older-style horror films, and become something of a cliche, if not quite as popular as werewolves and vampires.

In D&D, they are mid-powered undead, falling into what I've termed the "wilful corporeal" category, given that they retain sentience and intelligence alongside their physical body. Under that scheme, they would fall somewhere between a wight and vampire in terms of power, but they are quite different from both. The look of a mummy does not, of course, change notably between editions; they are bandage-wrapped mummified corpses, sometimes dressed in funereal finery, but more usually not. Like most D&D undead (except vampires) depictions of females are rare, although not quite to the extent that is true of, say, ghouls or wights.