Baneling

Baneling is the in-game companion acquired by purchasing StarCraft II: Heart of the Swarm Collector's Edition. Banelings are specially evolved Zerglings.

Source
Similar to past companions rewarded through purchasing the Collector's Editions, this item will be mailed to all characters on the account.

Pet Journal
''A rolling, explosive member of the Zerg swarm, the baneling deals terrible, terrible damage. Very dangerous in close quarters.''

Description
Players are mailed this pet by The WoW Dev Team:

Trivia

 * This is an homage to Starcraft 2.
 * Baneling, the Heart of the Swarm Collector's Edition reward, is based off the Baneling unit, known for dealing heavy damage when exploding. The battle pet has the themed abilities Centrifugal Hooks and Baneling Burst.
 * The phrase "Terrible, terrible damage" can be seen on its description, a reference to Dustin Browder, game director of StarCraft 2, who, during the demonstration matches of the game, used to use a lot this trited phrase.