The monster killed by Siegfried in Germanic mythology is known as Fafnir. Fafnir was originally a dwarf who transformed into a fearsome dragon due to his greed for gold. The heroic tale of Siegfried showcases themes of valour, betrayal, and the peril of unchecked ambition. By slaying Fafnir, Siegfried not only proves his bravery but also symbolizes the victory of good over evil. This myth highlights the consequences of greed and the complex nature of heroism in Germanic tales.