The dragon associated with guarding treasures and ancient secrets in myth is Fafnir from Norse mythology. Originally a dwarf, Fafnir transformed into a dragon due to his greed for the treasure he guarded, which was cursed. This creature is depicted as formidable and is slayed by the hero Sigurd, who seeks glory and riches. Fafnir's tale illustrates the dangers of avarice and the heroic struggle against overwhelming odds.