The Yallitians were a technologically-advanced species from the Delta Quadrant who had three spinal columns.
Neelix mentioned the Yallitians in 2371 when the USS Voyager came upon a planet rich in dilithium, stating "I bet there are a few Yallitian engineers who'd give all three of their spinal columns to know where this planet is." (VOY: "Phage")
This species was only mentioned in dialogue.