The plumed serpent of central Mexico: wind, wisdom, priesthood, and one very convenient legend.
Documented
Older than the Aztecs
The feathered serpent is carved at Teotihuacan centuries before the Aztec state existed, and appears at Tula and Cholula after. As icon and deity, the attestation is archaeology, not rumor.
Assessment: Documented in stone across multiple civilizations and centuries.
Stories
The exiled priest-king
The tales tell of Topiltzin Ce Acatl Quetzalcoatl, the pious ruler of Tula, tricked, shamed, and exiled east over the sea, promising to return. A king-sized ghost story with a calendar date on it.
Assessment: Told in post-conquest sources, mixing history, myth, and politics in unknown ratios.
Interesting claims
Cortés never played the god
The famous claim that the Aztecs mistook Cortés for the returning Quetzalcoatl appears thinly and late, swelling in colonial chronicles written after the fact. Modern historians, notably Matthew Restall, read it as a justification built backwards: if the god ordained it, nobody resisted.
Assessment: The strongest recent scholarship treats the returned-god story as post-conquest manufacture. It flattered conqueror and conquered alike, which is exactly why it survived.