People use the term legend or myth now to refer to religions other than the ones they believe in. Christians don’t call Christian stories myths and Muslims don’t use the term myth to refer to the Koran, but atheists might apply the term myth or legend to both.
Ancient Greece had a religious tradition long before writing was invented. This religious tradition included narrative accounts of the actions of gods and heroes. The term “mythos” simply means an account or story – and many of the stories we have concerning the Greek gods in Homer, Hesiod, the Homeric hymns and tragedy seem based on material derived from oral tradition, though other stories of the gods may be later literary inventions.
Aboriginal myths are entirely from oral tradition as writing was only introduced to most aboriginal cultures from outside after myths were well developed, although recent myth systems, such as the cargo cults, are sometimes associated with interfaces with literate cultures.