Jormungand

In Norse mythology, the sea serpent Jormungand was a child of Loki and Angerboda. The Aesir knew Jormungand would be dangerous for humanity, so they tossed him into the waters that encircled Midgard. Jormungand grew so big that he was able to surround the earth and grasp his own tail.(see Ouroboros).

During Ragnarok, Thor will finally kill Jormungand, but not before Jormungand can drop poison on Thor and kill him as well.

Alternative: Midgard Serpent