Giant rats appear to simply be larger versions of normal pre-War rats. The giant rats in Fallout: New Vegas appear to have lost most of their fur (likely to radiation) and have slightly-glowing turquoise eyes. Like normal rats, they also have very large incisors. Giants rats are often found in darker areas, such as Cerulean Robotics and Broc Flower Cave. Their meat is not used in any crafting.
Giant rats will not attack on sight, but will stand defensively in the way bighorners do when the player is spotted. Only when the player draws to close or attacks will it become hostile. Certain examples, such as the 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. Giant rats are also susceptible to and count towards the Animal Control perk. Occasionally they will make a sound that resembles a short howl.
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. 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 the player on sight along with any nearby mutated rats, unless they have the Animal Friend perk.
Every three days, three children in Freeside will be seen chasing a non-hostile giant rat. If the Courier kills the rat, the children will say "Thanks, mister/lady" and perform an eating animation on the rat (like the player will perform if he/she has the Cannibal Perk). 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. It is likely that these children are working for Genaro, as he mentions that he has his 'little helpers' who gather food for him. The children have knives, but they will never attack with them. Also, if you put the rat on a car or any other high place they will "eat" the rat but it looks like they're eating the object you placed the rat on. You can drag the rat making the kids trying to eat it, so you can lead them in a possible thug ambush.