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Wiki:

Jenny Greenteeth is a figure in English folklore. A river hag, similar to Peg Powler, she would pull children or the elderly into the water and drown them. She was often described as green-skinned, with long hair, and sharp teeth. She is called Jinny Greenteeth in Lancashire, but in Cheshire and Shropshire she is called Ginny Greenteeth, Wicked Jenny, or Peg o' Nell.

She is likely to have been an invention to frighten children from dangerous waters similar to the Slavic Rusalka, the Kappa in Japanese mythology, or Australia's Bunyip, but other folklorists have seen her as a memory of sacrificial practices.

The name is also used to describe pondweed or duckweed, which can form a continuous mat over the surface of a small body of water, making it misleading and potentially treacherous, especially to unwary children. With this meaning, the name is common around Liverpool and south west Lancashire