Species 262 was the Borg designation for a relatively primitive race native to the Delta Quadrant. They were assimilated sometime in the late 22nd century.
The oral history of Species 262 referred to a substance which could "burn the sky," which turned out to be the Omega molecule. The Borg first learned of the molecule from their assimilation of this species. (VOY: "The Omega Directive")
This species was only mentioned in dialogue.