Urban myths like, ufo's are extraterestrial aircraft,the chupacabra and Nessie exist in the shadow lands of possibility,it's not impossible that they might exist in some form,just highly unlikely.
The fact that the tales usually refer to unamed FOAFS means nobody can identify the subject of the story so they can never be be proved true or false even if it is plausable.
Myths are widely believed but usually based on misunderstandings of the facts, thus they are to all intents and purposes untrue.