Giant rats simply appear to 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 glowing turquoise eyes. Like normal rats, they also have very large incisors. Giants rats are often found in darker areas, such as Cerulean Robotics, broc flower cave, and Vault 11.
Giant rats will not attack on sight, but will stand defensively in the way bighorners do when the player character is spotted. Only when the player character draws too close or attacks them will they become hostile. Certain examples, such as the giant rats found in Freeside, who will not attack regardless of whether or not it is attacked. As with most mutated animals, giant rats will not attack if the player character has the Animal Friend perk. Giant rats are also susceptible to, and count towards, the Animal Control perk. Occasionally they will make a sound similar to a howl.
These look identical to fully grown giant rats, but are smaller than the standard giant rat. They usually don't attack but instead run away. However, if pups are encountered with normal giant rats, they will accompany them in the assault on the player character.
Found in the broc flower cave and New Vegas sewers, rodents of unusual size are the largest of the mutated rats found in the Mojave Wasteland to date. They are identical to their giant cousins, save for their increased size and attack strength. They are aggressive and will attack on sight, unless the player character has the Animal Friend perk.
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 "Sweet" Jill. Except for their size, they are identical in appearance to their smaller cousins, though are more aggressive and will attack on sight. If the Animal Friend perk is taken, like all animals, the unnaturally large rodents will not attack.
Every three days, two 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 Courier 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 and baseball bats, but they will never attack with them.
There is a unique dialogue option with Jane in Freeside during the quest Debt Collector if the Courier has the Wild Wasteland trait. They can ask her about the "R.O.U.S" in the broc flower cave where her raided caravan lies.