Raijū
Raijū (雷獣, "thunder animal" or "thunder beast") is a legendary creature from Japanese mythology.
Mythology
Its body is composed of lightning and with the form of a white and blue wolf or dog (or even a wolf or dog wrapped in lightning) being the most common, although it can be represented with other forms such as tanuki, fox, weasel, tiger, cat, bear, porcupine, boar or dragon. It may also fly about as a ball of lightning (in fact, the creature may be an attempt to explain the phenomenon of lightning, such as ball lightning). Its cry sounds like thunder.
Raiju is the companion of the 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. This prompts the Raijin to shoot lightning 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, but other legends say that Raiju will only hide in the navels of people who sleep outdoors.