Unfortunately, I don't not believe many of our 'mythological' creatures really exist, but I do think they are founded on some solid evidence, or at least creative reasoning, for the sciences and beliefs that were available at the time of thier incarnations. The dragon, for example, I think arose from the finding of dinosaur fossils and bones. Werewolves and vampires, from a dogmaitic fear of different cultures and superstition. Nessie and bigfoot/sasquatch/yeti , however, I think have the best chance of existing, sasquatch a little more so.
All of this is not so say that we should not embrace these gifts that many cutlures have passed down to us, though. I consider them a heritage of our imaginations, and they are more awesome than many mundane real-life things. I don't think they are any less enjoyable for being real or not.