Giant rat

Giant rats are simply an oversized version of these 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.

Small
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, grey and even white. They appear in the Core Region, as well as in Texas.

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

Fallout: New Vegas
Giant Rats in Fallout New Vegas are much weaker, and serve as an entry level enemy.They usually don't attack but run away. (NOTE: could someone please tidy this article up please?)

One can be found being chased by children in Freeside
 * Several can be found in Cerulean Robotics