I was going to guess you were wondering about setting myths.

Well, generally everything expanding one or two generations of any given race is going to be considered myths by that race. We're seeing it happen today with contemporary history, where there's a lot of thoughts of previously documented wars and we wonder what is true of any of the reasons given behind them and we start to romanticize them too. It is also happening to the first world war right now and we're seeing the coming generation not understand the lessons taught to us by the 2nd world war and repeating the same mistakes.

Uhm, but anyways, usually a lot of myths are presented in the setting details for whatever game you are running, but any DM worth their salt will be willing to allow general new myths to be established for the game as long as it does not conflict with other information.

Which can be really hard to pull off.