Morgan Le Fey originally appeared in Chivalric Romance in the Matter of France — tales of Charlemagne — before showing up in the Matter of Britain — tales of King Arthur. In fact, many characters from the Arthurian legend incorporate pre-existing legends, such as Tristan and Iseult.
Older Than Feudalism: The Classical tale of Jason and the Argonauts brought together many mythological heroes in a sort of Justice League Ancient Greece. "Hero" in Classical Mythology was an awfully vague term (as Larry Gonick put it, heroes aren't necessarily heroic, just excessive), and since the mission was to steal an item (the Golden Fleece) "Justice League" isn't a terribly good name for them....
Maid Marian, the shepherdess, featured in many pastoral plays popular at May festivities. Then there was a fad for Robin Hood plays. Then someone put the two characters together. It worked so well that Maid Marian is best known as a Robin Hood character, even though her independent existence predates him.