Mermaid Kidnapping

"If you go down to the water by yourself, the mermaids are gonna take you." Or, alternatively, "If you don't behave yourself I'm gonna throw you to the mermaids."

Summary

If a child goes to the shore, docks, or into the water without an adult to watch them, mermaids will come and take them away. They'll act friendly and happy, splashing and playing in the water to tempt children into diving in, but then they'll steal the child away and take them back to their home and never let them see their family again.

Historical Basis

Mermaids do actually exist, though they rarely show themselves above the water and even more rarely interact with humans. There have been a few cases of children being taken by mermaids, but these cannot be proven and, in reality, the children were most likely abandoned anyway. Nobody really believes that these or any other children were actually taken my mermaids. Most don't even believe that mermaids actually exist.

Spread

Almost every parent has said this to their child at some point, in whatever language they speak. There's no way to know who said it first or when, but with how people are constantly traveling the world, the spread was almost immediate.

Variations & Mutation

Some more violent cultures will say that the mermaids will kill and eat the children, rather than just taking them away forever. Some will also use a variation that threatens that the parent will throw the child to the mermaids if they don't stop whatever unruly behavior they are exhibiting. Exact phrasing varies depending on the language spoken.

Cultural Reception

Mermaids who know about the saying kind of scoff and roll their eyes at it. Sure, humans hunt them for their scales but they're the savages who need to be watched out for?