Skroggle infestations have been reported on over a dozen planets. The highly adaptable Skroggle physiology allows them to thrive in almost any athmosphere. Skroggles are omnivorous and their bizarre metabolism enables them to vomit plant-like creatures that excrete volatile acidic spores.
Gnaw was taken from his home-planet as a Skroggle specimen by Kremzonian scientists. But Gnaw was not a beast that could be caged: his acid-breath melted his holding pen and Gnaw started to rampage through the science facility where he was held. No-one knows exactly what followed, but the facility was found 12 days later, abandoned, and overgrown with some sort of weird, toxic flora.
Gnaw was eventually noticed by a passing band of Amphibian cosmic bikerpirates. Gnaw was in need of some friends, and he was taken in by them, nurtured and trained to fight in black-market exotic-alien arena fights. Occasionally his masters sell Gnaw's "services" to any war general that is desperate enough to set loose the rabid, chitinous menace upon the battlefield.
Source: The Developer Website.