Sy-O

Billions of years prior to the Battle of Hoth, this exogorth came into existence near a black dwarf star on the fringes of the Outer Rim Territories. Its father-and-mother gave it the name Sy-O, which, in their language, signified "the color of unloneliness." As a still-blind and feeble being, Sy-O absorbed the dying star's radiation before sealing its mouth for centuries, gradually developing its body. During this time its parents taught it the Hum and the ways of their species. Over time, Sy-O developed a basic atmosphere inside its body, and its sensory organs became sufficiently developed to perceive the Galactic Core, prompting it to embark on a lengthy journey towards it, a practice known as the Road of All Moons among exogorths.

During its billion-year journey, Sy-O observed the rise of both the Jedi and Sith Orders, the conflicts between them, and the Clone Wars. Sy-O also constructed its accretion sphere, a protective rocky shell for its delicate skin. Eventually, it stumbled upon a ship graveyard within the Ryloth system, where mynocks were feeding on the remains of starships; it consumed these creatures to enrich its internal ecosystem.

At one juncture, Sy-O, still a young exogorth barely over a billion years old, sensed the presence of a Clew, a gathering where exogorths meet to socialize and reproduce. Sy-O traveled to the Hoth system, where the assembled exogorths' accretion spheres were mistaken for an asteroid field, and joined others, including Si-Yy and To-X, in showcasing its internal inhabitants. However, the other exogorths were not impressed by the primitive nature of Sy-O's internal life, and they mocked and ignored it during the mating rituals.

Heartbroken, Sy-O attempted to sleep and dream within its accretion sphere, drifting aimlessly around the asteroid field. Humans mistook the sphere for an asteroid and named it "the Big One."

Seeking refuge from the Death Squadron following the Battle of Hoth in 3 ABY, Han Solo, Chewbacca, and their passengers Leia Organa, C-3PO, and R2-D2 piloted the Millennium Falcon into Sy-O's mouth, believing it to be a tunnel within the Big One. The ship's landing awakened Sy-O, who sensed the presence of the humans, including Leia's Force-sensitivity. Thrilled to welcome new inhabitants, Sy-O immediately began producing oxygen to sustain them within its body. It also sent out its resident mynocks to greet the humans in their new home, but Han and Leia, perceiving the mynocks as an infestation, attempted to shoot them. Sy-O was confused by their apparent hostility but continued to increase oxygen production. Meanwhile, it listened to their conversations, recognizing the bond between Leia and Han and pondering the nature of C-3PO and R2-D2, sensing that they were silicon-based like itself.

Eventually, the humans re-boarded the Millennium Falcon and attempted to escape the cave, which distressed Sy-O, who felt unworthy of being their host. Even as they flew out of its mouth, Sy-O tried to keep them inside, ejecting mynocks it thought were frightening them and even leaving its accretion sphere for the first time in centuries to try and swallow them once more. Abandoned, Sy-O mourned the loss of its newfound friends and tried to fall back asleep.

Shortly after, TIE fighter pilots from the Imperial Navy's Theta Squadron, searching for the Falcon near the asteroid field, attacked Sy-O. It attempted to consume the radioisotope-rich TIEs, destroying Dawn's in the process, but Shadow shot it, shattering one of its teeth and forcing it to retreat back into its shell.