D&D Monsters: Lycanthropes

In the real world, "lycanthrope" is just another word for "werewolf"; the word literally means "wolf-man" in Ancient Greek. In D&D, and many other RPGs, however, it's meaning has been expanded to refer to a general class of shapeshifters, of which werewolves are merely the most common example. While others have appeared in supplements from time to time, there are four core types other than werewolves. All of these operate in the same general way, in terms of how the moon, silver, and so on, affect them, and how they pass on the condition, so that what I've already said about werewolves typically applies to all of them, too.

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1E

Wererats

Wererats as such appear to be an invention of D&D, although some other legendary creatures (such as vampires) or evil sorcerers have often been said to be able to transform themselves into rats or similar vermin. Non-shapeshifting "rat-men" do predate D&D, but even they are a 20th century invention, appearing in stories by, among others, H.P. Lovecraft and Fritz Leiber. These are most likely the source of Gygax's inspiration.

Wererats are the lowest rung on the stepwise progression of 1E lycanthropes, although they already have the hit dice of a bugbear, the second highest creature in the 'evil humanoid' progression. Unlike the other lycanthropes they can, right from the beginning, transform into both rat and rat-man forms; the latter, unusually, retain human feet and regularly wield weapons. They live in communities of a dozen or so individuals and, according to 2E normally only breed with humans; the condition is passed on to the child only if the mother is the wererat (which makes sense for a bloodborne pathogen). 

D&D Monsters: Werewolves

Werewolves are a longstanding feature of European folklore, with references to men who can turn into wolves dating back to at least the Ancient Greeks, although the belief does not seem to have become widespread before the Middle Ages. For most of this time, however, werewolves seem to have been thought of as evil sorcerers or (in pre-Christian times) one-off individuals cursed by the gods. This "evil sorcerer" version most closely matches the sort of werewolf seen in the works of Tolkien and C.S. Lewis and is quite different from the D&D version.

That instead borrows from horror fiction, most notably the Universal Pictures Wolf Man films of the 1940s. Many of the tropes we associate with werewolves today were made popular by those films, and, in fact, often don't date back much further than the 19th century. In these films, however, as in some more modern examples such as An American Werewolf in London and Harry Potter, werewolves are portrayed as (mostly) tragic individuals, while in Twilight they seem relatively benign. D&D, like many other RPGs, makes them definitively evil although the potential for tragedy is still implicit in their ability to pass on the condition to others.