Quetzalcoatl, an ancient Mayan god, is of a race of giant aliens who rise out of the mist within the cavernous Mayan pyramid on P7X-377 when the crystal skull has teleported a visitor to a dimension in which they can be seen. He spoke the Mayan words, "Uy ah ual ing ual ing wetail," meaning "The enemy of my enemy is my friend." He is an enemy of the Goa'uld, and therefore a friend to Earth, and he welcomed SG-1 and Nicholas Ballard to the planet. He recognized Nicholas Ballard from having visited 29 years earlier, and invited him to remain to exchange knowledge and culture.
Voice of Quetzalcoatl: Christopher Judge
Episode Reference: Crystal Skull