Giant Sewer Rat
From Marcus "Bagman" Brown. You wouldn't want to meet this rat in a dark alley. Or anywhere else, for that matter. Critter type. Notice that if you use the caged version, you won't get achievement credit for .
The 
Giant Sewer Rat is a fishing catch from the Underbelly in Dalaran, with a very low drop rate, and is notable for being a P/P breed unlike most other rats and mice.
Guide awaiting review
Fish the Dalaran Underbelly
Head to the Underbelly in Dalaran and fish there — after 737 casts at a fishing skill of roughly 650 to 800, one player recorded catching 1 Giant Sewer Rat out of 737 total catches. If you run low on bag space, vendor your trash loot such as 
Sewer Carp, 
Magic Eater, 
Partially Eaten Fish, and 
Drowned Rat at Angelo Pescatore, found at the pier inside Cantrips and Crows. Note that 
Crystal Vial is the only item from fishing here that stacks above 20.
Understand the Achievement Requirement
Catching the Giant Sewer Rat directly through fishing awards the achievement I Smell A Giant Rat. Using a caged version of this pet does not grant the achievement, so avoid purchasing a caged one for this purpose. If you buy the item from the Black Market Auction House and use it directly, the achievement will still be awarded.
Know the Pet's Battle Stats
As a battle pet, the Giant Sewer Rat is a P/P breed, setting it apart from nearly all other rats and mice which lean toward speed. It has 14 attack power at level 1, giving it a higher damage stat than comparable rats, though it lacks their typical speed advantage.
Tips
Buying a caged Giant Sewer Rat is typically cheaper on the Auction House, but it will not grant the I Smell A Giant Rat achievement — only using the directly fished or Black Market Auction House item version will.
