Creature responsible for the selective process of the tellurian species. Essentially, what led to them having two heads, one of them non-sentient.
Way, way back in the timeline, before tellurians evolved better manipulators and developed more sophisticated cognizance, large clutches of tellurian ancestors would be born in communal hatcheries. A genetic defect (which would become hereditary) in one of the parents led to all of his/her offspring being born as conjoined twins. Just after hatching, most of them would fight with their other half, unable to cooperate. A few of them were born with twins that were mostly brain-dead.
Along comes the creature pictured… most of the younglings in the hatchery are gobbled up, too preoccupied with their twins to swim away. Those with nearly non-functioning twins (as well as a few who had succeeded in their attempts at post-hatching fratricide) were able to swim off.
How they continued to survive, spread their genes (complete with defect), and ultimately branch off into their own evolutionary path, is another story.
(yes, I know it looks like a penis)