Buffy.
Shame.
I find I like very non conventional monsters stuff that really catches players off guard with it's oddity. Anime coming from a culture not close to my own tends to have some off the wall creatures and concepts that I sometimes shamelessly use or adapt.
The magic system of "Darker Than Black" is an interesting take on sponsored magic, where the sponsor only requires that the recipient perform a very strange and specific "remuneration" to be able to use their equally strange and specific power. I recall one char that had to break his own fingers in order to make things ignore gravity to achieve a levitation like effect. Another had to drink the blood of children to create instant space like vacuums. There were lighter tone ones like eating cheeseburgers for super speed or dog earing all the pages in a book for wind like magic. But the dark ones caught my eye, especially since they don't make any sense and the chars who have them are just as clueless as to why as anyone else, this and moreover they were just kind of stuck with these powers randomly without asking.
So yeah Japan and the weird stuff it churns out is great material.
Also native american folklore has some truly terrifying ideas sometimes, a lot of them are generic but some are just... scary especially the ones that seem to have a foothold in reality. Case in point Wendigo. Both considered a monster and an affliction. Apparently the monster would call to you in a mind control kind of way and make you crave human flesh, you were then a Wendigo yourself. When Wendigo ate humans they gain their size and become bigger, thus increasing their appetite. This effect made it so that a Wendigo would starve even as it gorged itself endlessly, an uncontrollable hunger making it constantly hunt for humans.
Now the scary part, there are many a documented case of native americans inexplicably cannibalizing their fellows and claiming that they were Wendigo. Often times they would ask to be killed to stop themselves, which tells me they really did believe it and weren't just trying to get away with crime. One case involved a man who ate his family wife and 2 kids even when he could have gotten food reasonably easily. This strange Wendigo phenomena seems to have disappeared nowadays but... s*** man that is creepy.
So yeah monsters rooted in reality make for good nightmare fuel. I like to show my players the real world stuff too, that really changes the atmosphere of a session.