Guardian Cub

The Guardian Cub is a smaller, companion pet version of the Winged Guardian mount.

Source
This item was available from the Pet Store for $10 US or €10. It was released on November 1, 2011.

On June 4, 2014, it was announced that the Guardian Cub would be removed from the Battle.net Shop and into hibernation in the Blizzard Archive no earlier than June 18th, 2014.

Pet Journal
Still a little clumsy, this tiny winged lion is learning how to be the best mount it can be!

Letter
Players are mailed this pet by Breanni:

Notes and trivia

 * Unlike previous Pet Store pets, this item can only be sent to ONE character on your account.
 * This pet is only soulbound for the first 24 hours, after which it can be freely traded or auctioned to other players.
 * When you attempt to learn it, you are given a confirmation check: "Are you sure you want to use this item?"
 * Available only to players on realms located in North America, Europe, and Korea and requires a Battle.net account to receive.