Mole rat

This set of rodents just love to sink their teeth into your flesh. They vary greatly in strength — at the low end, you’ll find the rat very easy to defeat, while at the high end, the mole rat can pose a real threat. While the giant mole rats found on the West Coast are mostly covered with fur, the ones from the East Coast are not, possibly being mutations of different species.

Origins
On the "Pentagon Library Terminal" in the Citadel, it can be discovered that the mole rats in the Capital Wasteland (and possibly the other mutated rats in Fallout 1 and 2) are the result of a government program to develop an invasive organism that could be planted in China, where its aggression and fast breeding would help undermine the Chinese war effort. It is noted that a genetic 'kill switch' was included to help the US Army clean up the rats after the theoretical fall of China. (This may be the mechanism stumbled onto by Moira in the Repellant Stick.) Additionally, it is revealed that the researchers attempted to use FEV to further augment their creation, but dismissed this course of action as the result proved sterile.

Fallout, Fallout 2
Mole rats can often be found in packs together with pigrats and giant rats.

A rare variety of intelligent, albino mole rats exists. The only two known specimens are Keeng Ra'at and Brain. Mole rats are best killed by a combat shotgun to the face and are very easy to get a little xp from.

Immediate attacks
Specifically in Fallout 3, other than the Radroach, the Mole Rat will most likely be the first creature that you encounter, and also the easiest to kill. Melee weapons are an effective way to exterminate mole rats without wasting ammunition, particularly Sledgehammers and other weapons with high stopping power. Another good option, due to its lethality to Molerats, is the Repellent Stick, which makes fighting Mole Rats a breeze.

It should be noted that the Animal Friend perk works against mole rats, although there have been reports of bugged responses.

Appearances
Mole Rats appear in Fallout, Fallout 2 and Fallout 3. They were going to appear in the canceled Fallout: Brotherhood of Steel 2.