I wonder if people saw skeletons of dinosaurs and thus became the myth of dragons. Maybe unicorn was actually rhinoceros described by a far away traveller. Some images of unicorns were probably based on real animals, such as the one-horned rhinoceros or the narwhal—a small whale with a single long tooth or tusk that resembles a spiral ivory horn.
Fairies could have been dragonflies seen from distance. Gods, elves, gnomes could be actually very old ET sightings if we turn the mythology upside down and for a second think that mythology could come from true experiences sometimes and also if we speculate that there has been ET visitations here. Old myths at least are full of stories of "men from the sky" who access great powers of flying and great destruction etc.
Could be only stories though. I am not claiming anything just referring to a possibility with a universe vast in it´s time and space (time being thousands of years of mythological incidents here). This is pure speculative of course.
Sea monsters? Well, there has been seen (and catched)
http://en.wikipedia.org/wiki/Giant_squid
Flying spaghetti monster? Well, I am not going into that.
Santa Claus? Is Santa Claus real? I think it depends what you mean by Santa Claus. If you are referring to a
http://en.wikipedia.org/wiki/Santa_claus#Saint_Nicholas
Then he at least once was real.
We could also think that Mickey Mouse is sort of a real think in existence and even Santa Claus in a phenomena (not a real person, but a show person; actors play).
My point is: myths can be based upon real events. Even pink unicorns. What about goblins and all those small creatures in myths? Could they be early sightings of the typical "modern" ET.
I am not saying. Just speculating.