Filthy Slime
The Filthy Slime battle pet is learned from the Filthy Bucket which drops from Gol’than the Malodorous in Tol Dagor on Mythic difficulty.
The pet is personal loot and not guaranteed to drop, however it is cageable and can be bought/sold in the Auction House.
To get to the secret location and summon the boss follow these steps:
Enter Tol Dagor on Mythic difficulty and defeat the first boss.
Clear trash in The Drain and The Brig, and look for a Discarded Cell Key.
Open the cell marked in the screenshot below, go through the hole in the wall and at the end of the tunnel you’ll find the Filthy Bucket.
Click the bucket to summon Gol’than the Malodorous.
None the Wiser Guild 