"[...] those creatures don't exist, Mulder. They're folk tales born out of some collective fear of the unknown.”
– Dana Scully, The X-Files, Episode 71.
Personally, I believe they're just stories. Probably inspired by some weird guy who once got caught drinking a cheeky glass of blood. Over several decades, he became a fang-bearing, bat-transforming, mass-murdering immortal – with different cultures in different time periods having different interpretations. But hey, who knows? It's fun to speculate! =)