Fun fact time!
This is specifically related to the yokai called the kappa which looks like a humanoid turtle monster with a concave divot in the top of their skull that holds a small pool of water.
It's said that kappa reside in Japan's rivers and are used as a sort of boogeyman to warn little boys not to play by the riverside or else a kappa will pull them in and drown them.
Oh wait, actually, not only will they drown them, but they'll reach up inside their anus and rip out a mystical ball/organ called a "shirikodama" (small anus ball) that contains their soul and will eat it to gain its power.
Well, that's fucking weird, you might say, but it makes more sense once the context that Japanese culture intentionally removed over time is restored.
But before that, some other things about kappa:
They really enjoy sumo wrestling and will accept a challenge from anyone who asks.
They love cucumbers and an angry kappa can be appeased by offering them one.
Well, that's fucking weird, you might tell yourself again. But let's restore the original context of these bizarre creatures.
Before the Edo period and the west's influence on Japanese culture, homosexuality wasn't a big deal in Japan and it was a fairly acceptable practice for men to have male lovers on the side.
But western influence brought with it a huge stigma regarding homosexuality and Japanese culture eventually adopted it in kind.
Before this, kappa were seen as symbols of homosexual relations between men. They love wrestling with other men (sumo), they love cucumbers (a nod to their phallic shape), and are after the mystical ball inside a boy's anus (a nod to the prostate).
In short, the story about kappa drowning little boys wasn't to warn them about the dangers of playing by the riverside.
It was a warning not to get kidnapped, raped, and murdered by horny old men.
Such a concept existing in popular folklore didn't really mesh with new western values and these connotations evolved over time into bizarre curiosities about the kappa that fewer and fewer people were willing to openly discuss.