New Pet: Core Hound Pup

NEW Pet! According to WoW.com, many players received a Core Hound Pup in their mail upon logging in after the new patch. Blizzard has confirmed that the Core Hound Pup is sent to all characters (current and future) on all accounts tied to a Battle.net account that's linked to an authenticator:

The Core Hound Pup is a gift for anyone using an authenticator (attached to an account) whether it's the physical authenticator or the mobile authenticator. Anyone who attaches an authenticator will also receive this lovable new pet. Think of it as your own personal guard dog.

If you remove the authenticator from your Battle.net account, the Core Hound Pup will disappear from the collections of all of your characters after you log out of the game. You'll need to reattach an authenticator to receive the pet again, at which point you'll find him in your mailbox once more. If you switch authenticators while in-game, your pet will remain intact.