Every universe has its monsters, and Star Wars is certainly no exception. Along with the usual Sith Lords, there are actual monsters in space, some of which are hiding in asteroids and waiting to eat unsuspecting Corellian freighters on the run from the Empire. Others seem to be more reminiscent of classic kaijus, with Coruscant even experiencing a full on Godzilla level attack in Star Wars: The Clone Wars.
During the course of the Clone Wars, the Jedi and the Republic Army were called to Malastare to help the Dugs repel Separatist forces. Malastare was a key planet in the conflict due to its vast deposits of the fuel needed to run both sides' respective fleets. The Republic in particular required the support of the Dugs, as their fleet was dangerously close to running on fumes.
Fortunately, the Dugs were leaning towards the Republic, but they wanted the Separatists off of their planet first. Republic scientists brought an experimental bomb that would knock out all of the droids without causing any real damage, like an EMP. The pulse knocked out all nearby technology, and it gave the Clones and the Dugs plenty of time to take out the now incapacitated droid army.
As usual for Anakin Skywalker, this plan didn't go perfectly. The ground underneath the battlefield was unstable and collapsed, with several Clone Troopers falling in. Anakin went in to rescue them, only to discover a giant monster had been woken up. The Dugs called it a Zillo Beast, and it was reportedly the last of its kind, with the Dugs believing they had hunted the creatures to extinction.
These massive creatures were strong, tough and a lot more intelligent than the Dugs gave them credit for. They were also covered in thick scales that were impervious to any attack, including lightsabers. Seeing the potential in the Zillo Beast, Chancellor Palpatine ordered it brought back to Coruscant once incapacitated. This also made the Dugs happy, so they signed the treaty with the Republic.
Palpatine wanted the Zillo Beast killed and studied so that its impenetrable scales could be incorporated into his grand design; however, the Zillo Beast got loose and went on a Godzilla-esque rampage on Coruscant. In particular, it was on the hunt for Palpatine, seeing him as its main threat. This showed that its intelligence was greater than anyone expected.
The Jedi and the Clones worked together to save the Chancellor from his own bad idea while taking down what they thought was the last of the Zillo Beasts. It was incredibly difficult for the Jedi to do this, as their original plan was to take the Zillo Beast to an Outer Rim planet where it could be alone and live out its life. None of the Jedi involved were enthused about killing what was believed to be the last member of an entire species.
Palpatine ordered his scientist to clone the Zillo Beast, presumably to weaponize them. While the results of that plan have yet to be discussed, another Zillo Beast was eventually discovered on a planet in Wild Space, where it was worshipped like a god. Darth Vader tried to kill it, but he was unsuccessful. Kylo Ren returned many years later and finished what his grandfather had started. Whether or not that Zillo Beast was the result of Palpatine's cloning operations has never been revealed.
The destructive rampage committed by the Zillo Beast on Coruscant was unlike anything else seen in Star Wars. While there were several larger than life creatures, few of them took the fight to the core of the Republic. Given the hidden areas in the depths of that planet and the Emperor's plans to clone the Zillo Beast, there could easily be a secret installation filled with more just waiting to finish the job their ancestor started.