Lets determine the truth and then apply it to each example.
A possessed person loses his mind and can get supernatural strength from demons. I am always reminded of the Chinese legends. Like 'Crouching tiger, hidden dragon''. Then, if we go after demons we find them. They can reveal themselves as a werewolf in a bush. I have a Congolese friend I trust, who has some crazy stories of this.
Vampires possible, just no long life and never your own power. Werewolves as in you transforming, impossible. But seeing them late at night as part of demons tormenting you, possible.
Unicorns, maybe when they find fossils of them existing. Mermaids and fairies, only in Hollywood.
I find the Greek mythology story of Polyphemus the cyclopes quite interesting. I imagine a very big guy, blind in one eye and shunned from society.