The once-common Vanthian Terrier Rat was, at one time, a playful pet commonly owned by small children and ladies of mature and solitary dispensation, furry, faithful, and, while given to occasionally roll in unidentifiable decaying substances and enthusiastically eating its own (and others') feces, hardly the feared whispered name of latter times, capable of putting fear into the most stalwart of hearts.
The problems began when a certain Terrier Rat, known by his owners as "Basylle", was out for a nightly stroll and happened on a discarded bag of P'YooRhenia Hobling Chow, which the animal happily devoured every nibble of and trotted off home, happy as a giant mutant fire clam. What none realized was that the bag of food in question had been intended to be clad in lead and dropped to the bottom of the Icy Lake of Hori, having been contaminated by mysterious phasic radiations emitted by a glowing green meteor that had wiped out P'YooRhenia Factory #5 a few days before. One day later, after a severely disturbed night spent dreaming of what, none know, Basylle was now 10 feet long, easily massed two hundred pounds, glowing a rich shade of green, and had the oral equippage of an asteroid worm with a very, very good dental plan. Basylle thereupon devoured the family who owned him and the neighbors, an event which was described as "not typical" for him, proceeding then to eat a full third of the shire in which his owners had lived. At this point some of the more attentive of Vanth's defenders, fearing the worst, took it on themselves to capture or otherwise neutralize Basylle - whereupon it was discovered that not only was the little dear immune to magical, psionic or phasic energy, likely because of the fell radiations he had been exposed to, but physical attacks had no effect either, merely splitting off pieces of him which quickly grew into new Basylles. Thusly, one ill-considered air-strike by the Viraxian Air Force later, the single mutated Terrier Rat had become a horde of glowing omnivores in the broadest sense of the word, devouring all they encountered down to Vanthian bedrock.
It was only the quick dispersal of an aerosol form of the Deadly Potion of Ambrina that proved fatal to the Basylle horde, which had been moving in the direction of the Funfair Nomads (possibly attracted by the smell of B'Hlgn Waffles), and the once beautiful and fertile lands which had previously been the emerald gem of Vanth were now the poisoned and devastated Forbidden Wastes where live only the most resilient of organisms and peoples. The particular glowing meteor in question has never been located, and, considering the number of glowing meteors which present themselves on Vanth in a single month, in the interest of never again risking either the Infestation or the use of the Potion, the breeding of Terrier Rats has been banned in Vanth as a whole, save but for certain reckless individuals in the Shunned Towns who hope to create a controlled mutation similar to Basylle for use in pit fighting - this being one of the reasons the Shunned Towns have that appellation.