5.1 PTR: Pets From Vanilla Raids

UPDATE: The Pet Journal on the PTR lists where each pet will come from. We've updated the list below with information on which boss and raid will drop the pet.

Mr. Bigglesworth has changed from a dropped companion to a pet reward for completing the achievement to collect all the pets listed below.

A new achievement was datamined from the latest 5.1 PTR build, plus 12 new pets that correspond with it (plus one extra that's not part of the achievement)! [source]

The new achievement is called Classic Raid Collector Raiding With Leashes and requires you to collect all the pets listed under the achievement. As the name implies, these companions drop from raid bosses. They will be Bind on Pick Up.

• Anubisath Idol - Emperor Vek'lor (Temple of Ahn'Qiraj)
• Ashstone Core - Golemagg (Molten Core)
• Chrominius - Chromaggus (Blackwing Lair)
• Corefire Imp - Magmadar (Molten Core)
• Death Talon Whelpguard - Broodlord Lashlayer (Blackwing Lair)
• Fungal Abomination - Loatheb (Naxxramas)
• Giant Bone Spider - Maexxna (Naxxramas)
• Harbinger of Flame - Sulfuron Harbinger (Molten Core)
• Mini Mindslayer - Prophet Skeram (Temple of Ahn'Qiraj)
• Mr. Bigglesworth - Reward for completing Raiding With Leashes
• Stitched Pup - Gluth (Naxxramas)
• Untamed Hatchling - Razorgore the Untamed (Blackwing Lair)
• Viscidus Globule - Viscidus (Temple of Ahn'Qiraj)

Although 13 pets were datamined, only 12 will be from raid bosses.

Let's Get Ready to Raid!
Mumper confirmed on Twitter that the newly discovered pets will indeed drop from the vanilla raids—Molten Core, Blackwing Lair, Temple of Ahn'Qiraj, and Naxxramas. [source]

The droprate for each pet is described by Mumper as "not too bad", and he's confirmed that the companions from Naxxramas will be available from both 10 and 25 man.

There are still many questions surrounding the new pets. Which bosses will they come from? And will they be adorable miniature versions of those bosses?

We'll update with more information as soon as we find out!