Raijū (雷獣,"thunder animal" or "thunder beast") is a demon from Japanese mythology.
Its body is composed of either lightning or fire and may be in the shape of a cat, tanuki, monkey, or weasel. It may also fly about as a ball of lightning or fire (in fact, the demon may be an attempt to explain the phenomenon of ball lightning). Its cry sounds like thunder.
Raiju is the companion of Raijin, the Shinto god of lightning. While the demon is generally calm and harmless, during thunderstorms, it becomes agitated and leaps about in trees, fields, and even buildings (trees that have been struck by lightning are said to have been scratched by Raiju's claws).
Another of Raiju's peculiar behaviors is sleeping in human navels. This prompts Raiden to shoot arrows at Raiju to wake the creature up, and thus harms the person in whose belly the demon is resting. Superstitious people therefore often sleep on their stomachs during bad weather. Other legends say that Raiju will only hide in the navels of people who sleep outdoors.
Theory about origin
The myth of the thunder animal probably came about as a way of explaining the natural phenomenen of ball lightning.
Several creatures in anime and manga are inspired by Raiju, such as the Pokémon Raikou (Pokémon) and Raichu as well as the Clow Card The Thunder in Cardcaptor Sakura. The PS2 video game Shin Megami Tensei: Lucifer's Call also includes the Raiju as a recruitable enemy.