Youkai are supernatural creatures that originate from Japanese mythology. Can also be written as Yokai. Some Youkai have even been known to resemble humans in appearance. Creatures similar to Youkai are called Ayakashi.
Often in Japanese mythology these beings were infamous for their alien natures with some being dangerous to humans and almost all of them being beyond human understanding - they varied from spirits of the dead to possessed objects or lesser Deities and have become a prominent part of Japanese culture.