What a question! The history of myths is very complex one, so its better to concentrate on one particular story. Robin Hood for example. A great number of outlaw stories are merged together, with real people added to it (King John and the Sherriff of Nottingham) as well as characters from early stories, such as Maid Marion. In some stories the names may change, like for instance when the Romans began to adapt Greek stories.
...Not to mention Alexander Hislop's "The Two Babylons", which documents in fine detail how the early Catholic church threw out the simple factual accounts from the Bible and replaced them wholesale with the myths and legends of Greece, Egypt, and ultimately, Babylon itself, from where all the others had originated. Heavy going, sometimes, but worth every month of your life it will take you to read it thoroughly and understand it all.!! :o)