Lil' Ragnaros

Lil' Ragnaros, the Tiniest Firelord.

Source
This item is available from the shop or the In-Game Store for $10 or €10. It was released on November 29, 2010 along with Moonkin Hatchling.

Pet Journal
It's never too soon to summon this tiny aspect of the dread Firelord.

Your foes will be purged by fire with Lil' Ragnaros on your team.
 * In-Game Store

Taste the miniature flames of Sulfuron - ''Get your guildmates fired up with Lil' Ragnaros, the mini-elemental lord. Not only can he serve as a cooking fire, but he'll also sporadically ignite nearby critters that wander too close to his burning form. As a being of pure elemental fire, he will even submerge into the world's molten core when you start moving and then reemerge next to you wherever you stop.''
 * Blizzard Shop

Letter
This companion is a miniature version of Ragnaros, who mails it to players:

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.
 * Unlike most companions, Lil' Ragnaros is stationary unless his owner moves far from him. This causes Lil' Ragnaros to turn to smoke, and then reform next to the owner.
 * Lil' Ragnaros can also be used as a cooking fire. Note that this ability fades after about a minute, requiring him to be summoned again to reactivate.
 * In Heroes of the Storm, Ragnaros has a skin called "Lil' Ragnaros" in reference to the World of Warcraft pet.