Raijū (雷獣, Raijū) is a legendary creature from Japanese mythology.
Raiju is the companion of Raijin, the Shinto god of lightning. While the beast 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 and thus harms the person in whose belly the demon is resting. Superstitious people therefore often sleep on their stomachs during bad weather, but other legends say that Raiju will only hide in the navels of people who sleep outdoors.
Its body is composed of lightning and may be in the shape of a cat, fox, weasel, or wolf. It may also fly about as a ball of lightning (the creature might be an attempt to explain the phenomenon of lightning). It's cry sounds like thunder.
- Raijū on Wikipedia