Quetzalcoatl was a feathered-serpent deity worshiped by the Toltec, usually associated with or equated to Kukulkan.
Captain Kirk and the USS Enterprise discovered an alien entity that claimed to be Kukulkan in 2270. While Kukulkan's power appeared vast, he was in fact mortal and merely long-lived. (TAS: "How Sharper Than a Serpent's Tooth")
According to the CD-ROM game Star Trek: 25th Anniversary, the Enterprise encountered Quetzalcoatl himself in 2268. He appeared as a humanoid alien with supernatural powers and long life. He had made missions of peace to ancient Earth (similar to Kukulkan's visit) as well as Klingon colonies during the 2260s. His supernatural power stemmed from a gland at the top of his spine.
It would appear that Quetzalcoatl and Kukulkan were both members of an advanced alien race that visited Earth periodically and showed an interest in the Egyptian, Mayan and Aztec civilizations. It is possible that their races are the same race known as the Preservers.