Notes All of the listed pets can be obtained from bosses in Molten Core, Blackwing Lair, Temple of Ahn'Qiraj, and Naxxramas.
Raid Boss Pet
Molten Core Magmadar [Blazing Rune]
Molten Core Sulfuron Harbinger [Mark of Flame]
Molten Core Golemagg the Incinerator [Core of Hardened Ash]
Blackwing Lair Razorgore the Untamed [Unscathed Egg]
Blackwing Lair Broodlord Lashlayer [Blackwing Banner]
Blackwing Lair Chromaggus [Whistle of Chromatic Bone]
Temple of Ahn'Qiraj Prophet Skeram [Jewel of Maddening Whispers]
Temple of Ahn'Qiraj Viscidus [Viscidus Globule]
Temple of Ahn'Qiraj Emperor Vek'lor [Anubisath Idol]
Naxxramas Maexxna [Dusty Clutch of Eggs]
Naxxramas Gluth [Gluth's Bone]
Naxxramas Loatheb [Blighted Spore]
These pets have a rare drop rate.
Added in patch 5.1, this achievement was introduced along with the twelve battle pets. The reward is a battle pet version of Kel'Thuzad's cat, Mr. Bigglesworth.
Anyone complete this as yet?
I have 2/3 for AQ40, 1/3 for Nax, no attempts with MC (i hate MC) and BWL but i have 1 90 that can solo them all (monk) and 3 85s that should be able to clear all the bosses without any problems.
On another note you can learn the pet, cage it and sell it and you will keep your progress for the achievement (EX if you sell your mini mind slayer it wont remove it from the list of pets you have learned to get mr bigglesworth.)