While watching Millenium, we talked about the Ouroboros, as depicted to the right. It is a serpent devouring its own tail, and symbolizes multiple things, such as death, but also infinity. I personally knew a few ideas that presented the Ouroboros in other names, and decided to do some research to see where it else comes from.
The Ouroboros has shown up in many religions of cultures, from all over the world. Alongside being a serpent eating its own tail, frequently the story also includes it from being of the sea, or relating to the sea in some fashion. The story also has a lot to do with the combination of death and rebirth, as one may assume from the picture.
One of the earliest stories of this is Tiamat, the Goddess of the Salt Water, which also represents chaos. She's in the Babylonian creation myth, in which before the sky and the earth, there was only the ocean, Tiamat. Tiamat begot all the other gods, and thus was at the head of the chain (for those interested in Matriarchy, Goddess' like Tiamat and Gaia show women may have once been in control of the world). Paranoia broke out among the younger gods, and a bunch of murders likeable to a war occurred. Tiamat was displeased by this, and decided to drown the world (Great Flood anyone?), but Marduk cut Tiamat in two. Her separation became the heavens and the earth, and from her death all life came.
The one I know best is Jormugandr, form Norse mythology. Jormugandr is one of Loki's three children (four if you count Sleipnir), and has a large presence during Ragnarok. Jormugandr is described as a serpent so massive that it lives at the bottom of the ocean where the world is largest and goes across all of it to the point where its biting its own tail. As such, Jormugandr is known as the Midgard Serpent (Midgard = Norse World, so aka The World Serpent). When Ragnarok does finally come, Jormugandr will rise from the sea and poison the seas and skies. Thor will battle Jormugandr, and kills him by separating Jormugandr's upper jaw from his lower jaw to the point that it rips Jormugandr in two (*cough TIAMAT cough*. Unfortunately, in Ragnarok basically every God dies...and in the fight Thor is poisoned, and the stress of tearing Jormugandr asunder lets Thor only take a few steps forward. But, the tale of Ragnarok continues in that a few key Gods survive, and bring out a better world for us all. There are a few other Norse tales that also have large serpents biting their own tails.
While on the concept of Apocalyptic events...lets look at the Apocalypse. Leviathan is the sea serpent, and one of the enemies of man when the end comes. The word Leviathan means "Coiled," and being a Sea-serpent you get an image of a serpent biting its own tail. Some pictures of Leviathan depict him as such. I am actually much less aware of Leviathan's presence in the Apocalypse other than that seas will rise and help drown us out.
Well...so far it looks like Ouroboros is evil. Quetzalcoatl is the opposite, being one of the 4 major Aztec gods. Quetzalcoatl name symbolizes a flying serpent, and is the god of the sky. While not the god of earthly water, Rain (and as such floods) would fall under his domain and control. Quetzalcoatl's story is a little different, in that he helped created the world, and then he departed on a boat of snakes into the ocean (which if we know our American history, Spanish come back and are mistaken for their long lost God). Quetzalcoatl has been liked to Vishnu of Hinduism, who has also been described as Flying Serpent, and seen with the Ouroboros.
There's a bunch more too. Such as Hinduism having a serpent that circles the worlds ocean and helps keep the world at balance. A good amount of various African and Asian mythologies also have presented the concept of the Ouroboros in various forms.