Depths. It guards the area with the Sewer Chamber Key, and the slide-shortcut to the Blighttown entrance. The Spider Shield is also located on a ledge above, accessible from the shaft behind the Butcher.
This "huge" rat has an axe lodged in its left eye, from an earlier encounter, and is located in the first large antechamber in the Depths.
This Rat is fast and hits hard, but luckily there are escape routes which allows for safe replenishment of est.
It can also be attacked from above, if the player drops down from the shaft behind the butcher. Use ranged attacks, firebombs or a massive damaging falling attack for an easy kill.
Note: This may not work for the PC Version. I have not confirmed myself but a friend of mine has not be able to do so. If someone could test this theroy and note that it works it would help.