Type/Species: Great Serpent
Origin: Norse Mythology
Regin the dwarf told a man named Sigurd to slay a mighty dragon named Fafnir. What he did not tell Sigurd, however, is that Fafnir was once Regin's own brother. Fafnir killed their father in order to gain wealth and, with his greed, transformed himself into a mighty dragon so that he could protect his horde better. So, unknowingly, Sigurd rode with Regin to the dragon, taking along his broken-sword, Gram, which he inherited from his father. 
Sigurd knew, somewhat, about dragons and their hides. He knew that he could not penetrate the dragon's scaly body with any weapon, and so Regin suggested that they stab his under belly. They dug a deep ditch for Sigurd to hide in, and, as the dragon came out for his daily trip to the water hole, Sigurd shoved the huge sword, Gram, into the dragon's soft underbelly. 
After the dragon was dead, Regin cut out the heart so that he could roast it. After it was roasted, Sigurd overheard birds speaking and they told him that the dwarf was planning to kill him. When Sigurd looked at Regin, he knew it was true, so he cut off Regin's head. Then he claimed the cave's treasure for his own. 
This story is depicted in what is called The Sigurd Runestone. The serpent looks to represent corruption and greed, while his brother, Regin, is no better and is a symbol too. Sigurd, caught in the middle, emerges victorious, and he is also somewhat greedy, for, as the story states, he went to slay the dragon for fame. 
When you read the summary, you might lean towards thinking this dragon is a Western dragon. The tough outer layer of scales and soft underbelly are not limited to Western dragons, however. In art, this dragon is depicted with having a long, slim body. 
For more information on footnotes and references, please see the bibliography.