My roommate told of the Kappa, who are turtle monsters that live in japan. According to some, these monsters only live by rivers. His mom would tell him that if he played outside after dark, the Kappa would come to get him. His parents and grandparents would tell him this to keep him from wandering out of the house at night. He says that is a tradition for parents to tell kids this, and that Kappa are so widely believed in by children that there are Kappa statues next to some of the rivers. The tradition of telling this story probably came from concerned parents, and used the kappa as a way to scare their kids into not doing certain things, like playing in the river or going outside at night. The actual turtle monster aspect probably came from the numerous species of turtles that are native to Japan.