Tired of the same old pint-sized companions? Well, the new TCG expansion, Servants of the Betrayer, will contain "super common MMO loot cards", with an average of two such cards appearing in every box of boosters. The newest card of this type is Papa Hummel's Old-Fashioned Pet Biscuit. Feed one to your minipet and watch it grow to roughly the size of a hunter's pet! AWESOME!
But there's a downside: the loot is actually a consumable (obtained as a stack of 50). My hope is that you'll be able to visit Landro Longshot for a refill whenever you run out, but my guess is that once you use your stack, it's gone forever.
Read the official Upper Deck Article for more.
UPDATE: I originally posted that this loot would be available to all characters on an account. This was incorrect—a couple of my sources had it wrong. My apologies.