The Zillo Beast was a semi-sentient reptilian-insectoid species endemic to the planet Malastare. In ancient times, these gargantuan creatures preyed upon a native indigenous species, the Dugs. However, the Dugs gradually gained the upper hand and slowly eradicated the Zillo Beasts through various methods, including drilling for fuel in their habitats. By the time of the Clone Wars, it was widely believed that the creatures were extinct, although it was rumored that at least one had survived.
During the Battle of Malastare, the Galactic Republic deployed an electro-proton bomb which immobilized a substantial Separatist invasion force of battle droids. However, the bomb's tremors inadvertently awakened the last Zillo Beast which went on a destructive rampage. Fearing an apocalypse, the Dugs demanded that the Republic exterminate the beast to which the Republic reluctantly agreed. Seeking to obtain the creature for scientific research, Supreme Chancellor Palpatine and Jedi Knight Anakin Skywalker covertly arranged for the beast to be immobilized by several stun tanks, giving the appearance that it had been killed.
This action succeeded in both rendering the Zillo Beast unconscious, and appeasing the Dugs who believed it to be dead. In return, the Dug Council and the Republic ratified The Treaty of Malastare that allowed the Republic access to the planet's fuel reserves. The Zillo Beast was shipped to the galactic capital, Coruscant for scientific study but escaped after arrival, putting the lives of millions of residents at risk and forcing the Jedi into action. They eventually managed to kill the Zillo beast, but its body was kept and Palpatine ordered an attempt to clone it.
At 97 meters long, the Zillo Beasts were gargantuan creatures that dwarfed many other objects and most living organisms in both size and mass in their environment. They had a roughly serpentine body with a long tail that trailed behind its form. It uniquely possessed a third arm protruding from its back. Its plate-like armor was virtually impenetrable to explosives, blasterfire and even lightsaber strikes.
Despite these advantages, a flaw in its protective armor, a small unprotected gap between the plates, allowed some attacks to penetrate and harm the creature. On one occasion, this gap gave enough space for a well-placed energy beam from a stun tank to render the beast unconscious. A Zillo beast could also be killed by poisoning it with Malastare fuel. This fuel contained toxins that made it an effective method that the Dugs used for many years.
The eyes of the Zillo beast glowed a pale yellow-green at all times and its nostrils were located on the top of its skull.
It appeared to have some form of rudimentary intellect, capable of recognizing facial features when hunting for Palpatine, and also capable of recognizing the intent behind words, even though it likely could not understand their precise meaning. It also had a sense of vengeance, as it tore its way through Coruscant simply to find and kill Palpatine, who had ordered its death.
The Zillo Beasts were once the dominant species of the planet Malastare and preyed on the indigenous sentient Dugs in ancient times. Over time, the Dugs began drilling their world for fuel which gradually culminated in the near extinction of the species due to a loss of habitat. The ancient Dugs were known to have cataloged a list of weaknesses in the creatures, including their vulnerability to fuel due to its flammability and to poisons in the fuel lethal to the Zillo Beasts.
During the Sacking of Coruscant in 3653 BBY, the Sith Lord Darth Malgus visited the office of the then defunct Supreme Chancellor. In his office he found a statue he suspected was made of Zillo Beast bones. However, he had only heard rumors of the animals.
Eventually, it was believed that the species had been eradicated, though it was prophesied that one such creature still survived and would emerge in the future to destroy them. Thus, the Dugs passed down a manuscript through the generations in order to prepare their descendants for such a day. During the Clone Wars, Malastare faced the threat of a Separatist invasion in 21 BBY which sought access to its substantial fuel reserves.
In response, the Galactic Republic dispatched a sizable army of clone troopers to secure the key Republic world's fuel reserves and to recruit the Dugs as allies. This mission was overseen by the Jedi Order which sent Jedi Master Mace Windu and Jedi Knight Anakin Skywalker to lead the campaign. However, the sheer number of battle droids on the planet led to Supreme Chancellor Palpatine authorizing the deployment of a new weapon—the electro-proton bomb. The weapon successfully obliterated the droid army but inadvertently awakened the last Zillo Beast, which wreaked havoc among the survivors of the battle.
Panicked, the Dugs made preparations to slay the creature and demanded that the Republic assist them, otherwise, Malastare would not ratify the treaty that would allow its fuel reserves to be used for the war effort. However, Master Windu refused to support this action, as he regarded this as the murder of an innocent life-form. Despite his protestations, the Chancellor publicly supported the action, but secretly sought to capture the beast alive for scientific research.
A scientific adviser close to him intended to study the Zillo Beast's impervious hide and replicate it synthetically for the Grand Army of the Republic's benefit. At the suggestion of Skywalker, Palpatine agreed to conduct an elaborate attack designed to stun the Zillo Beast using particle cannon fire, which would penetrate the gaps in its armor and render it unconscious. Once stunned, the Zillo Beast would appear dead enough to appease the Dugs and allow for the signing of the treaty. Once in a coma, the creature was to be transported to an undisclosed peaceful world in the Outer Rim where it could be studied and then released back into the wild.
However, the Dugs were already prepared to slay the creature and began dumping fuel into the pit where it was located in order to kill it. This act only enraged the beast, driving it to emerge from its pit and it wreaked untold devastation on both the Dugs and Republic forces. However, the use of the stun tank's power beam enabled the Jedi to knock out the creature.
With the treaty signed, the Zillo Beast's body awaited transportation but Chancellor Palpatine ordered a change in plans. Instead of being taken to its secret Outer Rim destination, the scientists wanted the specimen studied in a contained environment. Thus, it was agreed for the creature to be taken to the galactic capital Coruscant, where preparations were made for its arrival.
After the Zillo Beast was transported to Coruscant, it was studied by Dr. Sionver Boll. Chancellor Palpatine oversaw the operation, and he ordered the doctor to kill it, so that they could extract the beast's scales for study. Seeing this from its prison, the Zillo Beast growled fiercely and glared at the Chancellor with unfettered rage. In addition, the Zillo Beast also sensed Palpatine's true nature as a Sith. The doctor reluctantly accepted her orders and produced gas from Malastarian fuel, which had, according to the Dugs, already been successfully used to subdue, and even kill, Zillo Beasts. As it was sprayed with the gas, the Zillo Beast went berserk at exposure to the gas, and managed to break out of its restraints, and escaped the building in which it had been held. It rampaged around Coruscant, killing hundreds, if not thousands, of innocents before charging towards the Senate, intent on revenge upon Palpatine for ordering its execution. Although the Beast eventually had the Chancellor's ship in its grasp, action on the part of Anakin Skywalker ensured that Palpatine escaped. The beast was killed by the Republic working with the Jedi, using flares to make it roar so missiles full of the poison could be shot into its mouth. The monster's huge body finally toppled from the Senate Building dead, paying the ultimate price for the mistakes of Palpatine and others in power. After its death, the Zillo Beast's body was sent back to the Doctor with her new orders—orders to clone the Zillo Beast.