Once, in a LARP, I played Isaac Asimov on a panel which was arguing whether vampires were real. It went something like this (modulo my memory): I asked the audience to define "vampire", and they said that vampires were creatures that lived by drinking blood.

I said that mosquitoes were vampires. So they said that vampires were humanoids who lived by drinking blood.

I said that Masai who drank the blood of their cattle were vampires. So they said that vampires were humanoids who lived by drinking blood, and were burned by sunlight.

I (may have) said that a Masai with xeroderma pigmentosum was a vampire. And so on.

My point was that vampires were by definition not real - or at least, not understandable - because any time we found something real and understandable that met the definition of a vampire, we would change the definition to exclude it.

(Strangely, some mythical creatures, such as vampires and unicorns, seem to be defined in a spiritual way; whereas others, such as mermaids and centaurs, do not. A horse genetically engineered to grow a horn would probably not be thought of as a "real" unicorn; a genenged mermaid probably would be admitted to be a "real" mermaid.)