Zoo is a rather generic term describing an aggressive deck featuring three or more colors, usually centered around red and green. Usually the third color is white due to it also being a color of small effective creatures. While black and blue may occasionally show up, they rarely do.
Most Zoo decks in Legacy feature a suit of small creatures which get better with lands of neighboring colors in play. Among these are Kird Ape, its near reprint Loam Lion or Wild Nacatl. Tarmogoyf, a creature usually 4/5 or 5/6 for only 2 mana, is also a mainstay in this deck. Additional creatures such as Grim Lavamancer, Gaddock Teeg or Qasali Pridemage either give some board control or add to the long game.
The creatures are usually complemented with a suite of red Burn spells such as Lightning Bolt, Chain Lightning and Lightning Helix which can either kill creatures standing in the way or just go to the opponents head to finish the game off.
4 Kird Ape