Giant rats appear to simply be larger versions of normal pre-war rats. The giant rats in Fallout: New Vegas appear to have lost almost all of their fur (likely due to radiation), and have slightly-glowing turquoise eyes. Like normal rats, they also have very large incisors. Giants rats are often found in dark areas, such as Cerulean Robotics and Broc Flower Cave.
Gameplay attributes
Giant rats will not attack on sight, but will stand defensively in the way dangerous bighorners do when the PC is spotted. Only when the PC either draws to close or attacks will the giant rat become hostile, and attempt to bite its attacker. Certain examples, such as the small giant rats found in Freeside, will not attack the player regardless of whether or not it is attacked. As with most mutated animals, giant rats will not attack if the player has the Animal Friend perk, and are susceptible to and count towards the Animal Control perk.
Variants
Giant rat
These are light brown color, with glowing blue eyes and white hair on its back. The standard giant rat is equal in size to a mole rat.
Be warned, as the giant rat is not a creature to be underestimated; it can kill a low leveled wastelander within 3 hits and its attacks will ignore damage threshold. At low levels it's advised to avoid this creature, however it has a low DT and can be gunned down if the legs are crippled; this is advisable because they are normally quite fast and attack in groups of 3 to 7.
The largest of the mutated rats found in the Mojave Wasteland to date, they are identical in appearance to their giant cousins, save for their increased size and attack strength. They are aggressive and will attack on sight. They can be found inside the Broc Flower Cave and also in the sewers beneath New Vegas.
These are uniquely large giant rats, with only two in existence in 2281. They are found in the East central sewers in the same room as Jill. They are identical in appearance, except for size, to their smaller cousins. They are, however, far more aggressive and will attack you on sight along with any nearby mutated rats, unless you have the Animal Friend perk.
Every three days, three children in Freeside will be seen chasing a giant rat (see Giant rat (Chased) for statistics). If you kill the rat, the children will say "Thanks, mister/lady" and perform an eating animation on the rat. The children will still "eat" the rat even if it was reduced to a pile of goo or ash. Eventually the rat will respawn and the children will resume their never ending chase.
The rodents of unusual size are named in reference to William Goldman's novel and film The Princess Bride.
In Fallout 2, when player speaks to Sajag in Klamath, in the dialogue there will be mentioned that Trapper Town has problems with ROUS - rodents of unusual size.
