Fungal Abomination
Drops from Loatheb. Loatheb's body is covered in small pods capable of spreading blighted fungal spores up to 1000 yards given proper wind conditions. Undead type. Fungal Abomination is one of the new battle pets added in Patch 5.1. It drops off of in This is one of the pets listed in the new achievement: Information on this is here.
The Fungal Abomination is a rare Undead battle pet dropped by Loatheb in the Plague Quarter of 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 the Plague Quarter. You can solo this raid at max level.
Defeat Loatheb boss
Defeat Loatheb, the final boss of the Plague Quarter, to have a chance at the 
Blighted Spore, which teaches the Fungal Abomination pet. The drop is not guaranteed, so multiple runs may be needed.
Learn or trade the pet
The Blighted Spore is Bind on Pickup, but once you cage the learned pet, the caged version becomes tradable and can be found on the Auction House.
Complete Raiding with Leashes
Collecting the Fungal Abomination along with the other eleven pets from Molten Core, Blackwing Lair, and Temple of Ahn'Qiraj completes the Raiding with Leashes achievement, rewarding the 
Mr. Bigglesworth pet.
Tips
The Fungal Abomination is an Undead pet with three healing spells that trigger on damage dealt, and its level 1 stats are 158 Health, 10 Power, and 12 Speed.
