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Its body is composed of lightning and with the form of a white and blue wolf (or even a wolf wrapped in lightning) being the most common, although it can be represented with other forms such as tanukis, foxes, weasels or cats. 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.
In popular culture
Two such creatures appear in the anime Naruto: Shippuden. Sasuke Uchiha forms a Kirin from lightning in a storm to attack Itachi Uchiha. Kakashi Hatake briefly forms a dog or wolf from lightning chakra to attack Pain.
One of the kaiju in Pacific Rim is given the name Raiju. Resembling a crocodile or a mosasaur, it has a three-part head armor that protects its wormlike real head inside. It is portrayed as a fast swimmer with powerful jaws, but possesses no electrical abilities.
The Pokémon Raikou is described as a Legendary beast with electric abilities that usually appears during thunderstorms. It appears as a yellow, tiger-like creature with a curly, purple mane and a blue tail resembling a lightning bolt. It's also similar to the Pokémon Manectric. From Bulbapedia:
"Raikou may be particularly based on the Thunder and Lightning gods of most cultures surrounding the world, mostly the 雷獣 raijū (lit. thunder beast), a thunder demon of Japanese lore and is said to be Raijin's companion. It is said to be a creature that could take the form of a tiger, monkey, blue wolf wrapped in lightning, and many other animals. Raikou is specifically based on its tiger form, while the Pokémon Manectric is based on its wolf form...and possibly a yōkai of Japanese lore, the 雷獣 Raijū, an elemental lightning creature that can take on the shape of various mammals, including a blue wolf wrapped in white and yellow lightning."