One of the biggest quandaries of the Zoo player is what sort of curve to employ and how much acceleration to use. Unlike 60 card deck constructed formats which allow 4 copies of a card cube Zoo cannot rely on consistently having a Wild Nacatl start, nor can it ensure all of its ramp critters contribute to the aggression with exalted. Many of the most exciting and powerful gold cards cost three or four mana which has a tendency to up the mana curve of the deck quite a bit beyond something like red deck wins or white weenie. This means you either accept your deck is a little more midrange and ensure it has reach and staying power or you pack it with a good quantity of ramp.
The second issue in Zoo is the one drop assortment for which people either tended to play lots of mana critters or they would play loads of beaters such as Kird Ape, Loam Lion and Goblin Guide to supplement the solitary Nacatl. With just mana critters you are far less able to apply pressure and are most vulnerable to early creature removal. Dumping down a turn four Llanowar Elves you have ripped of the top does little where as a Nacatl would be quite a bit more serious. If you however choose the polar opposite of this plan you are left with very linear draws and a much lower average power level. You have both increased the number of weak late game cards like Kird Ape and you have removed some of your powerful high end due to having less ramp to get you there. It turns out that a mix of the one mana ramp critters and the most powerful beaters is best in Zoo. This gives you loads more options in the early game and it allows you to play a decent number of more expensive cards which leads to higher average power level.
There are 4 main ways to build Zoo which are RGW, RGWb, RGWu and RGWub. Due to the high number of playables and easily includable groups of cards with powerful synergy Zoo decks of the same colours can look wildly different. I recently tried out my new strategies for solving Zoos problems in a RGWu and a RGWb version of the decks. Both were very streamlined and performed very well for me enough to warrant putting Zoo back on the tier one list of archetypes. I find the five colour version hardest to build in constructed as you have so much choice of cards that are hard to compare and extra things to include should you wish like Tribal Flames. The blue version I made first and it was a little less refined that the subsequent black version but both were true to my aim regarding one drops and mana bases which seemed far more relevant than which planeswalkers I chose to use or which extra colours to splash.
Zoo With Black
23 Spells
Bird of Paradise
Elves of Deep Shadow
Avecyn Pilgrim
Noble Hierarch
Steppe Lynx
Wild Nacatl
Lightening Bolt
Path to Exile
Plated Geopede
Lotus Cobra
Strangleroot Geist
Tarmogoyf
Dark Confidant
Lightening Helix
Vindicate
Lingering Souls
Knight of the Reliquary
Sprouting Thrynax
Kitchen Finks
Ajani Vengant
Bloodbraid Elf
Wolfir Silverheart
17 Lands
Bayou
Taiga
Savannah
Plateau
Scrublands
Overgrown Tomb
Stomping Ground
Temple Garden
Wooded Foothills
Windswept Heath
Verdant Catacombs
Marsh Flats
Bloodstained Mire
Stirring Wildwood
Raging Ravine
City of Brass
Zoo With Blue
23 Spells
Bird of Paradise
Avecyn Pilgrim
Noble Hierarch
Llanowar Elf
Orcish Lumberjack
Wild Nacatl
Lightening Bolt
Path to Exile
Punishing Fire
Lotus Cobra
Qasali Pridemage
Strangleroot Geist
Tarmogoyf
Knight of the Reliquary
Kitchen Finks
Edric, Spymaster of Trest
Trygon Predator
Shardless Agent
Geist of Saint Traft
Sarkhan Vol
Bloodbraid Elf
Wolfir Silverheart
17 Lands
Taiga
Savannah
Plateau
Volcanic Island
Breeding Pool
Stomping Ground
Temple Garden
Wooded Foothills
Windswept Heath
Misty Rainforest
Arid Mesa
Flooded Strand
Stirring Wildwood
Raging Ravine
Grove of the Burnwillows
City of Brass
The Punishing Fire engine is reasonable against weenie creature decks and some control type decks with lots of utility dorks but it is probably not worth the hassle and power reduction. Without many more ways to support the engine it is likely to be less beneficial to your game than just having a Karplusan Forest and an Incinerate instead. The blue version had some other reasonably silly cards such as Orcish Lumberjack and Sarkhan Vol. Lumberjack is a little random at the best of times and will go from being a useless 1/1 to being super Black Lotus and totally dominating games. The home of the Orc is red green where he is most consistent but I wanted to see if he would be useful in a Zoo deck with such a high forest count. Sadly my few games were very inconclusive and so I shall probably have to try again at some stage despite the grief I got from my opponents for including silly cards. Vol is hated by lots of people and generally thought to be pretty bad and while I concede he is not one of the best walkers I do quite like him. Ultimately I played him to wind my playmates up by beating them with cards they think are bad and not because he was the best card for the deck. In my defence this is one of the least damaging places to play a weaker walker. I wanted to play one walker in the decks to give me variation in threats and some late game reach but didn't feel it mattered too much which. I wanted to spend no more than 4 mana for my walker as the single five slot I was happy to play was dedicated to getting some playtime on the new Wolfir Silverheart. Koth is a bit needy of mountains, Chandra is very low impact as is Garruk Relentless and I play Garruk Wildspeaker in every deck. Elspeth is definitely the best but the double white cost was offputting and I also have played her to death and so took the opportunity to have a go on the less used gold walkers. Both did their roles as required and I am still unconvinced there is all that much to choose between Ajani Vengeant and Sarkhan Vol in a deck like this.
Almost all the other cards are mainstays and widely accepted as very powerful cards and need no real explanation or justification for why they are in the deck and why they help you win. Zoo might be very easy to goldfish but being a very standard style of magic deck it tends to end up having very interactive and complicated games with most archetypes. It is also hard to build with consistency being of such high value while faced with seemingly limitless options for insanely powerful cards. I like the deck both to play with and against, it feels fair and a great benchmark by which to test decks against.