Core Hound Pup

The Core Hound Pup is a core hound companion.

Source
This item is mailed out to all characters on accounts that have a Blizzard Authenticator or Battle.net Mobile Authenticator attached, including accounts that are attached to an authenticator in the future. Apparently, it represents the Authenticator as being the 'guard dog' of your account.

Ownership of this companion is also dependent on an authenticator: removal of an authenticator from your account will also remove the Core Hound Pup from all characters' collections in that account.