The most famous knucker or water demon was said to live in a pool at Lyminster, near Arundel in Sussex. This was one of many knucker holes in the flat areas between the South Downs and the sea, which were held in local folklore to be bottomless. Not the least of the odd things about them was that they were reputed never to freeze in winter nor to dry up in summer.
Though the Lyminster knucker lived in water, it could also fly, and so is often classed with other British dragon legends. It’s said to have rampaged through the area, killing livestock and local people (in the manner of such beasts in such stories, mainly maidens). One story says it was disposed of by a wandering knight to gain the hand of the local king’s daughter (so essentially the legend of St George and the Dragon); another that a local man, Jim Pulk, baked a large pie laced with poison and left it on a cart by the pool. After the knucker had eaten the pie, plus the cart and the horses, it swiftly expired and Pulk cut its head off. Unfortunately, he then carelessly imbibed some of the poison himself and died along with the beast.
Knucker is a modern spelling of the Old English nicor or nicker for an imaginary being that lived in water. Nickera appears in the Anglo-Saxon poem Beowulf to refer to a lake monster. The word is linked to similar monsters in several Scandinavian languages and to Nixie, a German term for a female water-elf or water-nymph. It might also be a source for Old Nick, one of many names for the Devil, and for the German Nickel for a sort of goblin that lived in mines and from which we get the name for the metal. (However, nick in the sense of making a mark, of stealing, being arrested, and other senses, probably comes from a different root, but as we don’t know which one, can’t say for sure.)