The term "zoo" comes from Magic The Gathering and this was the name for an efficient and cheap creature deck with packs savannah lions, kird ape and so.
For HS, zoo is the name of the warlock's deck. All other fast decks with cheap and efficient creatures are usually called rush (hunter rush, shaman rush and so). THis kind of decks are also called face decks, because you always wants go to the enemy hero's face ;)