Zoo Warlock is one of the oldest decks in Hearthstone.
The deck rose to prominence in the game's early beta, way back in 2013, after it was built and publicized by streamer and now-Tempo Storm owner Andrey "Reynad" Yanyuk. It was immensely popular, mostly due to how cheap to build and easy to play it was.
The strategy was pretty simple. Play cheap minions, flood the board, and win with an aggressive strategy. The original included vanilla cards like Young Priestess, Dark Iron Dwarf, and Shieldbearer. As new expansion came out, cards like Haunted Creeper, Nerubian Egg, Imp Gang Boss, and Imp-losion helped it become one of the best decks out there.
Over time, Blizzard stopped printing insane Zoo cards. After Whispers of the Old Gods brought out Darkshire Councilman and Forbidden Ritual, it was out of favor as far as new cards went.
But that hasn't stopped players trying to bring it back, and now a new version of the deck is rising up the meta.
This list comes from decorated pro Dima "Rdu" Radu, but the origins of this new form are sort of unknown. Canadian player "Canaderek" brought it to DreamHack Austin last month to the astonishment of the casters, albeit with cards like Bloodreaver Gul'dan and Glinda Crowskin.
Rdu's list focuses on the minion-based strategy of the past, but all of the versions have identified a new key strand—healing. Considering how much of the deck does damage to your own hero healing is something that's been tried before, but the particular impact of Happy Ghoul makes it very strong.
Happy Ghoul and Lightwarden are both in the deck to take advantage of healing, with Voodoo Doctors, Fungal Enchanter and Lifedrinker in there to activate the effects. The current impact meta cards of Saronite Chain Gang and Fungalmancer are there to help the deck sustain past the first few turns.
Prince Keleseth is also key to the deck. Once upon a time not playing two drops in Zoo would be unthinkable, but outside of Demonfire, Dire Wolf Alpha and Vulgur Homunculus there aren't any good options anymore. The buff from Keleseth is definitely worth skipping those cards.
If you want to play something a little bit retro before the new expansion comes out, Zoo could be the deck to try. It beats the control decks in the meta as well as Odd Paladin, so it could bring you some ladder success too.