As the prophesized slayer of Thor, the Jormungander is definitely a top contender for the biggest ass-kicker among dragons. The Jormungander, along with two other freakish monster siblings, was the result of the unfortunate pairing of the god Loki and the giantess Angrbooa. Odin approached situation in the same manner a rural grandma would deal with unwanted kittens and put the three in a sack and tossed them in a river (actually it was the ocean).  In this environment the Jormungander grew until it was long enough to encircle the earth.  The Jormungander is said to be completely wrapped around the planet gripping its own tail.  When it finally lets go, it will be the beginning of Ragnarok.  At Ragnarok, the Jormungander will spew enough poison to contaminate the earth’s entire atmosphere. Thor will confront and slay the Jormungander but then will die from the beast’s venom.