The species apparently exists in a different phase of visibility that Humans cannot naturally see. To compensate for this, they release massive amounts of neutrinos into the area, allowing humans to view them. In form, they appear as a mist like species. The Omeyocans are enemies of the Goa'uld, greeting each visitor to their planet with the words Oo ya waaling waaling wey tayil—"the enemy of my enemy is my friend" in Mayan. They visited Earth in the past, apparently before Ra conquered the planet, leaving a crystal skull behind to communicate with the Tau'ri.
Nicholas Ballard, the grandfather of Daniel Jackson, discovered the skull and was transported to P7X-377. Years later, Daniel, his grandson, and the rest of SG-1 visited the planet once more, but as he stares in the eyes of the crystal skull, he was temporarily out of phase due to Teal'c firing a Zat'nik'tel at it when it starts to emit light. After completing the process, the Aliens came, and repeated their mantra. Daniel realized that they were referring to the Goa'uld, and the Omeyocans allowed Nick to stay. (SG1: "Crystal Skull")
The Omeyocan, named after a Nahuatl term for "place of duality", are an ancient alien species that warred with a group of evil exiled Goa'uld. Many of the race were killed in battle while the majority of their enemy were destroyed leaving only a single Goa'uld Queen called Coatlicue.
The remaining members of the race would journey to Earth where they developed the Mayan and Aztec cultures. When the Goa'uld were enslaving Humanity, the Omeyocan created an enclave of worlds that would be free from the Stargate network and allow those civilizations to develop in peace. These worlds would develop into the Tollan, Orbanian, Yaxkin and Xalótcan people.
Many of the writings within the Omeyocan's pyramids matched that of the enigmatic Furling species; whether the Omeyocan are Furlings or are an automated system developed by them is not truly known.
Behind the scenesEdit
The unpublished Fallout: Stargate Season Three sourcebook for the Stargate SG-1 Roleplaying Game developed by Alderac Entertainment Group named the giant aliens as the Tuohocan, though this was later overruled by the Stargate SG-1: City of the Gods novel, produced by Fandemonium, which named them the Omeyocan.