Coulbabunar was a Thylacoleo (locally known as drop bear) that served as Gunmonga's animal companion (and thus one of Shredder's and Kyknos's past lives). Because he was fated to be Gunmonga's animal companion, Coulbabunar was born at the exact same time as him. At the time of her birth, Coulbabunar was only around the size of a cheeto and completely helpless, spending all of her time in her mother's pouch, drinking milk. When she was around two months old, she noticed that her mother had suddenly stopped moving and the milk had stopped flowing.
The next thing Coulbabunar would notice was the hand of a kangaroo reaching into the pouch and pulling her right out of it! Coulbabunar was still tiny and helpless at this time, only around as big as a banana. The kangaroo was a hunter who had killed Coulbabunar's mother with the intent of selling her fur at his village's market, unaware that Coulbabunar's mother was carrying a joey in her pouch. Since killing baby animals was taboo in the kangaroo mountain tribes, the kangaroo, realizing Coulbabunar would die without a mother, took Coulbabunar back to his village, intending to raise her and release her back into the redwoods when she was old enough.
The kangaroo had an infant son named Gunmonga exactly as old as Coulbabunar, resulting in the two growing up together and forming an inseparable bond. When Gunmonga's father was killed during a raid on his village, the responsibility of raising Coulbabunar fell on the then 8-year-old Gunmonga. Gunmonga was forced to take up hunting himself in order to feed his growing pet, taking down creatures as large as bunyips (diprotodon) to feed Coulbabunar.
When Gunmonga turned sixteen, his life changed forever when his village shamans informed him that he was the next Man of Light and Shadow, and that it was his job to master Astral and Shadow Magic and defend peace and balance in the galaxy. Gunmonga would spend the next ten years travelling the galaxy, mastering Astral Basics, Astral Life, Astral Rock, Astral Lightning, Shadow Basics, Shadow Fire, Shadow Water, and Shadow Death. Coulbabunar, more than large enough to ride by this point, was Gunmonga's loyal steed all the way, helping Gunmonga both on and off the battlefield. Coulbabunar was loved by Gunmonga and his allies, and feared by his enemies, with the mere sight of her once putting a group of jaguar warriors on Aztlan to flight.
Ten years later, however, Gunmonga was recalled to his home planet of Atlantea with grave news: vikings from Ryu 108 were attacking Atlantea with a massive fleet of longships. Coulbabunar returned with Gunmonga, and the two tried to lead a temporary alliance of all of Atlantea's kingdoms and tribes in battle against the invaders. Despite Coulbabunar's mighty roar providing an intense morale boost for the Atlanteans, they were simply too disorganized to fight the vikings effectively, and were annihilated.
However, the vikings invaded Atlantea during a solar eclipse, which triggered Shadow Mode (a state where the Man of Light and Shadow's powers are increased exponentially, their eyes glow sea-green, their skin glows golden, and they lose all control of their actions). As a result, Gunmonga summoned a massive flood that completely destroyed the viking fleet. However, once Gunmonga reverted to his normal state, the floodwaters simply kept rising beyond his control. No matter how hard Gunmonga tried to use Shadow Water to rein in the rising tides, there was nothing he could do.
For the next year or two, Gunmonga rode around the planet on Coulbabunar, helping the Atlanteans build colossal glass domes around their flooding cities. Despite their best efforts, though, this was not enough and all Atlanteans who did not live in mountainous areas were forced to drown or flee the planet, eventually forming the Ishga civilization on the planet Ishga. Eventually, the floodwaters stopped rising, but the damage had been done and everything on Atlantea but the highest mountains were underwater. The Kangaroo Mountains that Gunmonga and Coulbabunar once called home were now Kangaroo Island (a misnomer since it's an archipelago not an island).
Overwhelmed with guilt from causing the flood that drowned most of his home planet, Gunmonga took Coulbabunar with him and went into exile deep in the desert on the planet Eaglypt, feeling that he had failed as Man of Light and Shadow and that he did not deserve the celebrity status that the position gave him. Gunmonga became a desert hermit, getting a new life in an extremely isolated oasis farming village, assuming a false identity as a refugee from the Atlantean flood.
The only times Gunmonga would leave his new small house were to go to the market to buy food for himself and meat for Coulbabunar. Coulbabunar was essentially reduced to a giant housecat, never being allowed to leave the house because that would give away Gunmonga's identity. The galaxy soon began to believe that Gunmonga was dead (and in Gunmonga's eyes he was), as no one had seen him since the Atlantean Flood. However, nobody could find the successor, causing a state of unease around the galaxy.
Gunmonga remained on Eaglypt for over 400 years (the Men of Light and Shadow tend to have much longer lifespans than normal people, and since Coulbabunar's life is tied to Gunmonga's, he lived this long as well), at which point he heard that a new fleet of vikings, even larger than the one that attacked his home planet, was coming for Eaglypt. At first, Gunmonga refused to come out of hiding, too ashamed of his own power to go into battle. As a result, Eaglypt's army was annihilated, the Eaglyptian capital was sacked, and he soon heard news that the vikings were heading directly to his village.
With nowhere to go, Gunmonga (whose health was already starting to fail him at this point), decided to own up to his past and confront the vikings. However, he could not defeat the vikings' whole army if he couldn't enter Shadow Mode at will and control his actions while in that state. Gunmonga then got on Coulbabunar's back and rode to a temple deep in the desert run by Anubian priests who knew the secret. He explained his identity to the priests, who shamed him for turning his back on the galaxy. However, they did not turn him away, instead calming down and saying that he had "lost his way" and that he must learn not to be ashamed of what he could not control.
Once Gunmonga stopped being ashamed of the power Shadow Mode gave him, the priests taught him how to enter the state at will and how to control it. After this, Gunmonga hopped on Coulbabunar's back and rode out into the desert to confront the viking army. When he found the viking army, he climbed off Coulbabunar as she let out a massive roar. Then, he entered Shadow Mode and used Astral Rock to summon a massive sandstorm that knocked the viking ships right out of the sky, but he was able to control the sandstorm and force it to subside once it had done its job.
However, once Gunmonga dropped out of Shadow Mode, the sandstorm had taken such a toll on his old body that his heart simply gave out and he died where he stood. Coulbabunar, whose lungs had been lacerated by the sandstorm, died seconds later.
Their bodies were returned to Kangaroo Island, where they were buried in Atlantea's Shadow Temple deep in the redwood forests of the Coorabar Alps.
Over 15,000 years later, Weasel was on Kangaroo Island, searching for the submarine parts needed to follow Tate to the Lunarium. During a dream, he was visited by the ghost of Gunmonga, causing him to wake up and go to Atlantea's Shadow Temple, which he unfortunately discovered was buried underground after millennia of disuse. However, Weasel managed to access it using Astral Rock. Once inside the temple, Weasel simply looked at a statue of Gunmonga and lost consciousness.
While unconscious, Gunmonga visited him in a dream, introducing him to the concept of the Man of Light and Shadow and explaining that he was one of Weasel's past lives from millennia ago. Weasel, still feeling some of the immense guilt for the damage he caused as The Pendragon a few months earlier, found comfort in Gunmonga's explanation of his life story, realizing that he could not control his actions while his soul was trapped in The Pendragon's body, so like Gunmonga eventually came to terms with what he did in Shadow Mode, Weasel should stop feeling so guilty about what The Pendragon did to the people of Arturia. Even so, after their meeting, Weasel did feel some reasonable doubt about the whole Man of Light and Shadow thing, but Gunmonga's explanation was later confirmed to him by The Norns.
A few months later while traveling around the South Galaxy in preparation for his confrontation with Tate, Weasel decided to make a stop on Atlantea to communicate with Gunmonga. However, when he arrived at the ruins of Gunmonga's temple, he did not see an apparition of Gunmonga.
Instead, he saw a ghostly apparition of Coulbabunar. Although Weasel drew his sword and prepared to attack, Coulbabunar let out a confused whimper instead of charging at him. In response, Weasel cautiously went up to Coulbabunar and started stroking her fur, causing Coulbabunar to playfully roll around on the temple floor. Coulbabunar eventually led Weasel into the inner sanctum of the temple, where the latter passed out upon seeing the eyes of a statue of Gunmonga glow sea-green.
While unconscious, Weasel envisioned himself on the Plane of Existence with Coulbabunar and Gunmonga also present. After petting Coulbabunar, Gunmonga explains the concept of the Man of Light and Shadow's animal companion to Weasel before Weasel woke up and noticed that he can see a little bit of Shredder and Kyknos in Coulbabunar's eyes.