Type/Species: Great Serpent
Origin: Scandinavian/Germanic Mythology/Norse mythology
About Nidhogg / Niddhogg / Nydhogg / Nidhoggr
Nidhogg is said to be the dragon that lives in the underworld Niflheim in Norse mythology. The name itself translates into things like "dread biter" and "striking full of hatred." 
Nidhogg lives by the root of Yggdrasill the World Ash, in the pit Hvergelmer. There he feeds on the dead bodies at Hvergelmir, or the bubbling cauldron, the spring in the Niflheim, which is also the source of the rivers of the world.
Should the Nidhogg not be feeding, he has several other tasks that he performs throughout the year. The first is gnawing at the root of the Yggrasill, and thereby joins with four stags called Duneyr, Durathor, Dvalinn, and Dainn, whom all graze on the tree from above. He also takes the time to send taunting messages to the eagle that proudly perches herself at the top of it. For this task, he uses the squirrel Ratatosk, and, in turn, the eagle sends Ratatosk to taunt Nidhogg. 
When the Ragnarok takes place it is said that the dragon will rise and take the dead with him in order to join the battle. The dragon will live through the battle and help lead the new order. 
For more information on footnotes and references, please see the bibliography.