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 nature, 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 dieties and have become a prominent part of Japanese culture.
In the Kingdom Hearts Unlimited Saga, Youkai are divided into two categories: Echthroi Youkai which are formed from negative energy mixed with either deceased human spirits or darkness; and Fair Folk Youkai which are actual deities and minor guardian spirits that originated as members of the Third Race allied to King Oberon or his mother Queen Mab.