Grunty the Murloc Marine is a companion that was given out to attendees of BlizzCon 2009, purchasers of the DirecTV pay-per-view of the event, and purchasers of the internet streamed video of the event.
Grunty will be delivered by in-game mail to every character on a player's battle.net account that has been attached to a Grunty code, much like collector's edition companions. If you merged your World of Warcraft account with your battle.net account after purchasing the stream, you will not get the pet.
He will attack nearby zergling when they spawn.
The letter delivered with Grunty is from Master Handler Sylvester: