Nidhoeggr (Pendant in silver)

Symbol of Norse Mythology, 13 x 41 mm, 925 Sterling Silver plated

In Norse mythology, Níðhöggr (Malice Striker, often anglicized Nidhogg) is a dragon who gnaws at a root of the World Tree, Yggdrasill.

Prose Edda
According to the Gylfaginning part of Snorri Sturluson's Prose Edda, Níðhöggr is a being which gnaws one of the three roots of Yggdrasill. It is sometimes believed that the roots are trapping the beast from the world. This root is placed over Niflheimr and Níðhöggr gnaws it from beneath. The same source also says that "[t]he squirrel called Ratatöskr runs up and down the length of the Ash, bearing envious words between the eagle and Nídhöggr [the snake]."

In the Skáldskaparmál section of the Prose Edda Snorri specifies Níðhöggr as a serpent in a list of names of such creatures:
These are names for serpents: dragon, Fafnir, Iormungand, adder, Nidhogg, snake, viper, Goin, Moin, Grafvitnir, Grabak, Ofnir, Svafnir, masked one. (Faulkes translation, p.137)

Snorri's knowledge of Níðhöggr seems to come from two of the Eddic poems: Grímnismál and Völuspá.
Later in Skáldskaparmál, Snorri includes Níðhöggr in a list of various terms and names for swords.