Giant rat

Giant rats are an oversized mutants of the Old World's common rodents.

Biology
Belonging to the superfamily Muroidea, giant rats are the result of Forced Evolutionary Virus. Fallout Bible 1 Fallout Bible 0: "3. Are the radscorpions a product of the FEV virus? When you talk to the doctor, Razlo, in Shady Sands, he tells you that they were once American Emperor Scorpions but that he has no idea how they mutated because radiation alone couldn't have done it.  For that matter, what about all of the other creatures of the wasteland?  Which ones have been mutated by FEV and which ones haven't?  Maybe in the bestiary each creature could have a stat that shows it's level of FEV infection." "Answer:
 * The radscorpions are a result of a combination of radiation and the FEV virus, and Razlo in Shady Sands is correct - they were originally Emperor Scorpions that have grown... big.
 * FEV-Infected Critters in the wasteland include almost all the ones you've seen in F1 and F2: Mantises. Most species of rats. Gecko lizards. Brahmin Scorpions Ants Various varieties of plants, including the Venus Flytrap. Rumor has it some dogs were affected, but no one's seen any, so for now that's just rumor. Of course, the centaurs are a mash of human, dog, and various other parts... but hey, who knows how that mutation came about. Grey was probably messing around in one of his labs.
 * Creatures not mutated by FEV probably did not survive the aftermath of the Great War, with the possible exception of cockroaches... and perhaps normal ants, though there are FEV-infected versions of these species.
 * No one knows where those bird noises in Vault City came from.
 * I'll try to include a chart of FEV-infected creatures in future additions as well as other critters that you may not have seen in F1 and F2. Most likely a great majority of insects were affected (they tend to breed much faster, and their mutations tend to become evident pretty quickly as the generations advance), possibly beetles, some spiders, cockroaches, and other creatures." They vary greatly in size, from ones the size of a cat to ones larger that the average human, like their mole rat cousins. Their greatest asset is their agility, which they use to quickly get the better of their prey and attack with their large incisors.

Rat


The smaller rats are still big enough to rival a dog in size. They usually attack in large groups and could visually appear out of nowhere, usually indicated by small pinpoints of red appearing in the shadows. They differentiate in color, being black, grey and even white. They appear in New California, as well as in Texas.

Giant rat


The bigger version is equal in size to a mole rat. As a result of radiation exposure, it became a little bigger and a little stronger than the smaller version.

Giant rat pup


Young giant rats are about the size of a small dog. They are not very resistant and are not very dangerous.

Midwest giant rats


These rats are even bigger than the mole rats and humans. Their visible spine gives them a disturbing appearance, while their size and power could prove a challenge for an inexperienced wastelander. They can be found in the Midwest and Texas.

Rodent of unusual size


The largest of the mutated rats found in the Mojave Wasteland to date, they are identical to their small and giant cousins, save for their increased size and stats. They are aggressive and will attack on sight. This is a cultural reference to the film "Princess Bride" in which the character "Wesley"/"Dread Pirate Roberts" fights off a giant rat, after replying to a question about their existence.

Unnaturally large sized rodent


These are uniquely large giant rats, with only two in existence in 2281. They are identical in appearance to other giant rats in the Mojave Wasteland, except for size. They are, however, much more aggressive and will attack you on sight along with any nearby mutated rats.