The Draco maritimus immensis, is a giant serpent which dwells around the edges of Discworlds, holding its tail in its mouth.
As well as the sea-troll Tethis passing within a few leagues of one in The Colour of Magic, an alternate universe in Equal Rites revealed in prism contained: "another slowly turning disc, surrounded by little stars. But there were no ice walls around this one, just a red-gold thread that turned out on closer inspection to be a snake - a snake big enough to encircle the world. For reasons best known to itself it ws biting its tail.".