"Faffner"
Faffner guarding the gold
Public DomainFaffner guarding the gold - Credit: Arthur Rackham

Faffner is a figure from Norse mythology, the son of a dwarf king. He turns into a dragon in order to guard a horde of treasure.

He appears in Wagner's Ring cycle as one of the giants who built Valhalla for the Gods and was paid off in treasure (including the magical ring).  As a dragon guarding the treasure, he is killed by Siegfried, who then takes possession of the cursed ring.