Giant rats are an oversized version of the Old World's common rodents. They vary greatly in strength, from ones the size of a cat to ones bigger than humans, like their mole rat cousins. They can be annoying - the rats are hostile and will begin combat if you draw too near, severely delaying the player's passage through rat-infested areas. The species that exists in the Mojave has acquired white hair, blue glowing eyes, a light brown color and is bigger than the normal species.
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. They differentiate in color, being black, gray and even white. They appear also in New California, as well as in Texas.
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, Texas and Mojave Wasteland. The Fallout 3 version was cut from the final release.
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 reference to the film "Princess Bride" in which the character "Westley"/"Dread Pirate Roberts" fights off a giant rat, after replying to a question about their existence.
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.