According to Wikipedia:
Raijū (雷獣,”thunder animal” or “thunder beast”) is a legendary creature from Japanese mythology. Its body is composed of lightning and may be in the shape of a cat, fox, weasel, or wolf. The form of a white and blue wolf (or even a wolf wrapped in lightning) is also common. It may also fly about as a ball of lightning (in fact, the creature may be an attempt to explain the phenomenon of lightning). Its cry sounds like thunder.
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. This prompts 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.