Fafnir (ᚠᚨᚠᚾᛁᚱ) is a dwarf who later turns into a dragon from the Icelandic Völsunga Saga. Fafnir his two brothers Regin and Ótr were the three sons of the sorcerer Hreidmar. Ótr had the like of an otter during the day and when the three Aesir gods Odin, Loki, and Hœnir were traveling they came across the otter. Loki killed the otter with a stone.
When the gods came to Hreidmar's dwelling they were proud to show off the otter's skin but Hreidmar and his remaining two sons were not happy and seized the gods. Holding them captive they released Loki and told him to gather a ransom by filling the otter's skin with gold.
Loki who was very clever filled it with the cursed gold of Andvari as well as the ring Andvaranaut which both were told to be items that would kill whoever possessed them.
When Loki presented the gold to the three Fafnir taken by greed killed Hreidmar to get all of the gold for himself. But due to his greed the curse turned him into a dragon or serpent with the only purpose of guarding his treasure. Fafnir would breath poison onto the lands around him making it impossible to go near his precious treasures.
Regin however plotted vengeance on his brother and recounted the story to Sigurd who later killed Fafnir.
Richard Wagner later retold the story in his epic play The Ring of Nibelung in which a few things are changed. The biggest change in Fafnir is that he is now a giant instead of a dwarf.