Jormungand

Like his siblings Fenrir and Hel, the venomous Jormungand was foretold to bring about great misfortune and was thus tossed into the sea. There, Jormungand thrived and grew so large he encircled the entire world, thus earning the name Midgard Serpent ("Midgard" being Norse for 'world,' specifically the human's world).

Jormungand often came into conflict with the god Thor. The giant king Utgarda-Loki turned the serpent into a gigantic cat and challenged Thor to lift him, an impossible task even for Thor though the god made impressive effort. Jormungand encountered his foe again when Thor went fishing, catching the snake using a cow's head as bait. The two began to fight but were stopped by Thor's assistant Hymir.
The Midgard Serpent and God of Thunder will get their chance to duke it out in the Ragnarok. Thor will fell Jormungand with his hammer, but will himself fall to the serpent's poison immediately after.