Lil' XT

Lil' XT is a miniature clockwork giant.

Source
This item is available from the shop or the In-Game Store for $10 or €10. It was released on April 15, 2010 along with Celestial Steed.

As of August 23, 2012, any players who subscribed to the World of Warcraft Mobile Armory got a free Lil' XT until the Armory itself became free. On August 14, 2017, with the release of StarCraft: Remastered, this pet was included in the korean-only.

With the See You Later Bundle on December 20, 2019, this pet was available at a discounted price with other items. On January 19, 2019, the pet is no longer available for purchase as it entered hibernation in the Blizzard Archive.

Pet Journal
Modeled after the huge titan prototype in Ulduar, this volatile little construct plays hard with its toys.

Everybody's ready to play when Lil' X.T. is around.
 * In-Game Store

Don’t break it this time - ''Programmed with boundless energy and a child-like enthusiasm, Lil’ XT, the mini-Deconstructor, awaits adoption by a capable and adventurous parent. Despite his diminutive size, Lil’ XT can still fracture the ground beneath his feet with his tiny tympanic tantrums that discharge massive amounts of electricity into the air.''
 * Blizzard Shop

Letter
This companion is a miniature version of the XT-002 Deconstructor. Players are mailed this pet by Jepetto Joybuzz:

Quotes
See XT-002 Deconstructor for quotes and sound files.

Notes and trivia

 * Available only to players on realms located in North America, Europe, and Korea.
 * Players who purchase it receive a code similar to that of the loot cards. When you enter the code in your account all characters are mailed a copy of this pet, as well as any future characters you create.<
 * Will attack Clockwork Rocket Bot (both types) and Crashin' Thrashin' Racer Controller (both types)
 * Acts like a Wind-Up Train Wrecker.
 * It will randomly speak some of its larger counterpart's lines. This initially played for all nearby players, but was quickly hotfixed to play only for the pet's owner.