Llyn Cwynch - Snowdonia
Llyn Cwynch is near Dolgellau and is part of the Nannau Estate and folklore says that the serpent who lived in the lake hunted both humans and animals, which it killed with its stare before eating them. However, whilst it was sleeping it was found by a shepherd who beheaded it. There is a cairn that commemorates the place of its death called Carnedd-y-Wibber (the Cairn of the Women's-milk-fed Serpent).
From Snowdonia and North Wales