Stitched Pup
Drops from Gluth. When Gluth died to a band of brave adventurers, these lil' guys were found in the corpse, gnawing on giant, undead bones. Undead type. Pro tip if your a dk clearing out the raid for pets. While the boss is possible to solo on 10 man even with his devour zombie ability it can take a while, in blood spec anyway.
The Stitched Pup is a companion pet dropped by Gluth in Naxxramas, introduced in patch 5.1 as part of the Raiding with Leashes collection.
Guide awaiting review
Enter Naxxramas Raid
Travel to Naxxramas and make your way to Gluth. The pet drops from both the 10-man version of the raid, so enter on either difficulty and defeat Gluth solo at max level.
Defeat Gluth for Drop
Kill Gluth and loot the corpse for the Stitched Pup. The drop rate is not guaranteed at 100%, but based on player data across multiple runs it appears to be roughly 1 in 3 boss kills, so repeated weekly runs may be needed.
Complete Raiding With Leashes
Collecting the Stitched Pup alongside all other pets from Molten Core, Blackwing Lair, and Temple of Ahn'Qiraj completes the Raiding with Leashes achievement, which rewards 
Mr. Bigglesworth.
Tips
The Stitched Pup's slot 3 ability, Consume Corpse, requires a dead ally to activate, making it ineffective when Stitched Pup is the first pet in a battle. It can be replaced by Plagued Blood at level 20.
