Sewer Beast is a level 50 rare crocolisk that is unique not because of his skin, but because of his spawn location. He is the only tamable rare that spawns inside of a capital city. Because he spawns in such a highly populated area it can make finding this guy alive quite difficult. His name is also likely a reference to the urban legend stating that alligators can be found living in city sewers.
He does not have the traditional rare spawn loot table. He has a 100% chance to drop a grey item; there is a chance he will drop other items, but it is very very minuscule.
He spawns in five different locations in the Stormwind sewers and patrols in the water within each patrol area. He does not move between the patrol areas once spawned. I have indicated each location in red on the map below.
Sewer Beast can be found with NPCScan and Silverdragon. You will need to add it to NPCScan though, his NPCScan ID is 3581.
You can also use this macro to help find and tame her:
/tar Sewer Beast
/cast Tame Beast
I found the Sewer Beast while not actually looking for him. I was listing some items on the auction house in Trade District when my Silverdragon alert went off. When I flew out this was what I found: