Raiju (or Raijū), meaning thunder animal or thunder beast, is a legendary creature from Japanese mythology said to be companion of Raijin, the Shinto (i.e., an ethnic religion of Japan) god of lightning.
Descriptions of Raiju appearance vary, ranging from dog to squirrel, but it’s more often seen as a wolf or badger. Its celestial form is that of a wolf wrapped in lightning, and it is said that its cry sounds like thunder.
While Raiju is generally a calm and harmless creature, during thunderstorms it becomes agitated, it falls from the sky with the lightning, and jumps from tree to tree, to fields and even buildings. After the storm has passed, evidence of the Raiju’s presence can be seen in the torn, deeply gouged tree trunks and woodwork where Raiju dug its claws in. Raiju-damaged trees are much sough after, felled and harvest for bark, for the Raiju-touched bark cures toothaches.
During storms, Raiju will also pounce on anyone taking shelter beneath a tree and enter any unprotected house. However, it cannot pass through mosquito net, so house windows, doors, and chimneys are often covered with net to prevent the entrance of the beast. This creature also hates the smell of incense, which is often lighted before and during a storm.
Raiju loves to sleep in human navels. This prompts Raijin to shoot lightning arrows at it, to wake it up, harming the person in whose belly the creature is resting. Because of this, tradition dictates, it's vital to keep one’s navel protected during a thunderstorm, by sleeping face down.
Despite their elusive nature, the physical form of Raiju has been captured on multiple occasions. The Edo period (i.e., between 1603 and 1868 in the history of Japan) in particular has many such cases. Once, Raiju got tangled in the ropes of a well and was taken alive. To be later exhibited in a brass cage in the Temple of Tenjin in Matsue. There, it took the shape of a badger that slept during fair weather, but would become active during storms with its eyes flashing.
References:
-Foster, M. D. 2015. The Book of Yokai. University of California Press, Berkeley.
-Hearn, L. 1910. Glimpses of Unfamiliar Japan. Bernhard Tauchnitz, Leipzig.