Raiju is a creature in Japanese mythology- singular, there's only just one of him- and is a shape shifting, electricity-spitting, navel-gazing monster made out of raw lightning.
He's also the pet of the Shinto god of lightning, Raijin, making it the most badass pet ever.
Raiju can take on the shapes of several things, including a cat, a weasel, a fox, a wolf, or a ball of living electricity. Keep in mind that he's still made out of lightning in these shapes excluding the wolf. For the wolf, he usually just goes as a blue and white wolf surrounded by lightning.
Typically Raiju is pretty mellow, but when a thunderstorm kicks in, he gets all hot and bothered and goes running around forests, leaping from tree to tree. Since his claws are, again, made out of lightning, this causes the trees to break or burn. When a tree is struck by lightning, that's because Raiju just bounced off of it onto the next one.
Also, sometimes he sleeps inside people's belly buttons.
I. . . don't know.
Because Japan. That's why.
So if you plan on sleeping outdoors in the near future, make sure to sleep with your stomach to the ground, or else you'll wake up with a lightning monster in your belly button. That's not actually the worst bit, either, since in order to wake his pet up, Raijin will zap Raiju with a bolt of lightning. Which is fine and all for Raiju, but probably won't end as well for the person whose belly he's in.