Nidhogg

Nidhogg (also spelled Níðhöggr or Níðhǫggr) is a Dragon who lives at the roots of the world tree, Yggdrasil. He is always chewing on the roots of the world tree trying to kill it. Ratatosk constantly delivers insults to him to try and destroy the tree faster than Vedrfolnir.

The Sword of Summer
Although he does not physically appear, Blitzen mentions Nidhogg to Magnus Chase after they were chased out of Yggdrasil by Ratatosk.

Trivia

 * Sometimes, Nidhogg is depicted as a serpent.