The raijū (“thunder beast”) is a creature of Japanese mythology whose body is made of, or wrapped in, lightning, and it is the companion of the thunder god Raijin. It most often appears as a white or blue wolf or dog, but is also drawn as a cat, fox, weasel, tanuki or monkey, its form varying by region. Usually calm, it grows agitated and leaps about during thunderstorms.
The raijū is said to sleep inside human navels, so Raijin shoots arrows at or strikes the navels of sleepers — which is why people sleep on their stomachs during storms. That an indifferent elemental force loosed by the heavens should nest inside a human body connects to our world, which keeps calamity as something arriving from a sealed source.