Moonkin Hatchling

Source
This item is found inside a Moonkin Egg, which is available from the shop or the In-Game Store for $10 or €10. It was released on November 29, 2010 along with Lil' Ragnaros.

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

 * Moonkin are blessed by Elune and spread nature's grace wherever they travel. The festive fowls are always in the mood for a dance.
 * The distinct roar of the moonkin is one of the most terrifying sounds in nature.

No matter which side you are on, it’s adorable! - ''Fresh from the nest, the Moonkin Hatchling arrives in your mailbox wide-eyed and ready to explore the world. Featuring unique Horde and Alliance versions, this adorable hatchling occasionally plants flowers at your feet and gladly /dances with any willing partners.''
 * Blizzard Shop

Letter
Players are mailed this pet by Lunaclaw: