The creature in Greek mythology known for guarding the golden fleece is the Colchian Dragon. It serves as a significant obstacle in the myth of Jason and the Argonauts, where Jason must retrieve the fleece to claim his rightful throne. The dragon is often described as fearsome and magical, symbolizing the challenges and dangers inherent in heroic quests. Its defeat not only signifies overcoming adversity but also the pursuit of knowledge and glory in the heroic narrative.