The Zillo Beast, sometimes called "the Beast," represented a non-sentient reptilian species that was native to the world of Malastare. The Dugs had long believed this immense creature to be extinct before 21 BBY; however, during the Battle of Malastare, the last known living member was accidentally brought to the surface by an electro-proton bomb deployed by the forces of the Galactic Republic.
Supreme Chancellor Sheev Palpatine intended to harness the Zillo Beast's impenetrable armor for the benefit of the Grand Army of the Republic. Soon after the creature's arrival on Coruscant for scientific examination, it managed to escape its confines and went on a destructive rampage throughout the capital city. Ultimately, the Jedi and the Republic military were forced to eliminate the Zillo Beast using a potent toxin, the same substance that had initially driven it to wreak havoc on Coruscant, but administered in much larger quantities. Following its death, Palpatine gave the order for it to be cloned. Initial cloning attempts were unsuccessful until after the establishment of the Galactic Empire and the acquisition of Kaminoan cloning technology, at which point doctor Royce Hemlock finally achieved a breakthrough, successfully cloning several beasts that were then transported to and contained within Tantiss Base inside Mount Tantiss.
A solitary Zillo Beast existed in Wild Space, where the Benathy people revered it as a god. However, it was eventually slain by Kylo Ren, the leader of the Knights of Ren and a military commander within the ranks of the First Order.
The Zillo Beast was an enormous creature resembling a serpent. Reaching a height of 97 meters, it dwarfed all other species on its home world of Malastare and held the position of apex predator until its supposed demise. Its long, whip-like tail featured eight prominent spikes, and it uniquely possessed a third arm extending from its back. Notably, juvenile Zillo Beasts lacked this third arm, which only began to develop as they matured. The Zillo Beast possessed two bright green eyes that granted it the ability to see in complete darkness. However, these eyes were absent until it reached adulthood. Furthermore, it had strong claws and powerful jaws capable of biting through metal. Its lengthy tail, resembling a whip, was adorned with large spikes.
Its thick, plated armor provided immunity to explosions, blaster fire, and even strikes from a lightsaber. During the conflict with what was thought to be the last of its kind, Jedi Knight Anakin Skywalker discovered that vulnerabilities existed within the armored plates, which could be exploited to inflict damage or incapacitate the creature. In the course of the battle, clone troopers managed to subdue the beast by targeting these weak points with RX-200 Falchion-class assault tanks, inducing a state of sleep that allowed for its transport to Coruscant.
During its time in captivity on Coruscant, Doctor Sionver Boll speculated that the creature might possess sentience. This conclusion was supported by the creature's aggressive behavior towards Supreme Chancellor Sheev Palpatine, whom it had witnessed ordering Boll to terminate it via hologram. After escaping its containment facility, the creature terrorized the planet in a failed attempt to assassinate the Chancellor, recognizing his face both on a holo-board and later within the Senate Building. It was eventually killed by poison gas deployed from Republic Gunships as it scaled the Senate building, but Palpatine gave Boll instructions to clone it.
Zillo Beasts sustained themselves by consuming electrical energy, as demonstrated by the electro-proton bomb that awakened the supposedly final Zillo Beast from its dormancy. They were capable of feeding on various sources of electricity, including those found in starship power conduits and planetary power grids. Direct contact with a potent electrical source would trigger immediate regeneration of damaged limbs and promote growth until they reached their maximum size.
Zillo Beasts were among the most formidable of all reptiles and were deemed impossible to tame. They exhibited a strong preference for solitude, only interacting with other members of their species when breeding became necessary. Following the birth of their offspring, one parent would typically raise the young while the other would return to its solitary existence.
Components such as the scales, teeth, and tail spikes of a Zillo Beast were utilized as decorative elements within the Ubrikkian Floating Fortress belonging to Garnac's hunting guild.
During the Imperial Era, the Galactic Empire made use of Sionver Boll's research and ultimately succeeded in cloning one specimen, utilizing the original's DNA and technology acquired from Kamino, along with the expertise of Chief Scientist Doctor Royce Hemlock. This specimen was transported to the headquarters of the cloning program located on Mount Tantiss on the planet Wayland, where it was confined within an underground facility and it managed to escape during transit on Silla, but was quickly recaptured. It was subjected to further experimentation in the facility, involving weapons similar to the RX-200 Falchion-class assault vehicles, with the aim of weaponizing its genetic material. However, it was released by the genetically altered clone Omega as a distraction for the Empire during an attack led by Clone Force 99 to rescue her, the clone prisoners, and a group of Force-sensitive children on Mount Tantiss, where they killed Hemlock to save her.
Around the same period, a Zillo Beast was venerated as a deity by the Benathy people on their native planet Benath. During a conflict between the Galactic Empire and the Benathy, Darth Vader proved unable to defeat the creature. However, years later, his grandson, Kylo Ren, killed the beast by diving into its mouth and destroying it from within. With their god vanquished, Ren compelled the Benathy to pledge allegiance to the First Order.
The Zillo Beast made appearances in the episodes "The Zillo Beast" and "The Zillo Beast Strikes Back" from the Star Wars: The Clone Wars animated television series. Star Wars creator George Lucas believed that the Zillo Beast would have an aversion to Palpatine and would pursue him. Dave Filoni jokingly suggested to Lucas that the Zillo Beast could sense Palpatine's identity as a Sith Lord and was, in fact, attempting to warn the citizens of Coruscant.
When questioned about the potential return of the Zillo Beast in Star Wars Rebels following the cancellation of The Clone Wars, Filoni stated in an interview with StarWars.com that such a comeback was not feasible due to the production challenges posed by its scale and the absence of a logical reason for the Galactic Empire to clone it. While he entertained the idea of introducing a juvenile specimen rather than a mechanical one, no Zillo Beast appeared in the show before its conclusion in 2018. Despite Filoni's earlier remarks regarding the lack of justification for the Empire to clone the Zillo Beast, the Star Wars: The Bad Batch episode titled "Metamorphosis" revealed that the creature was, in reality, a subject of the Empire's cloning program.