As far as I can tell, the oldest folklore monster in history is the revenant. The English Abbot of Burton wrote about two around the year 1090, although he didn’t call them revenants.
The Wikipedia page doesn’t list what I consider the best example of the revenant, a 2009 film. It shares the same title as the 2015 film, but the 2015 film tells the story of someone who should have died and didn’t.
There are more examples in fiction, the latest being the TV incarnation of “American Gods“. I read the book it’s based on years ago, a few years after it was published. The book is still sitting in one of my closets.
I used to like reading horror fiction. I also used to like watching horror movies. These days, horror stories turn me off. The television and movie industries have milked vampire and zombie stories to death, pardon the pun.
I’m more interested in the history of the monsters depicted in horror fiction. More precisely, why did people think someone was a revenant or a vampire? Were they seeing the effects of some disease or were their minds playing tricks on them? Did superstition take over when no other reasonable explanation existed?
Filipinos are very superstitious, even those who’ve lived in urban areas all their lives. Don’t make fun of a Filipino who truly believes in certain creatures or you could end up with a lifelong enemy. I’ve been smart enough to keep my mouth shut for a long, long time.