I enjoyed playing Apex Predators in 2012, so I'm also building on what I learned from playing that deck. First, the decklist:
Creatures - 25
1 Ulvenwald Tracker
3 Garruk's Companion
1 Vinelasher Kudzu
2 Primodial Hydra
1 Dungrove Elder
3 Leatherback Baloth
1 Chameleon Collosus
1 Master of the Wild Hunt
1 Obstinate Baloth
3 Garruk's Packleader
2 Indrik Stomphowler
1 Vorapede
2 Primalcrux
1 Primeval Titan
1 Rampaging Baloths
1 Terra Stomper
Other Spells - 12
3 Prey Upon
2 Primal Bellow
1 Lightning Greaves
4 Nature's Lore
2 Cultivate
Land - 24
24 Forrest
Apex Intro: back in DotP 2012, there were 2 things I really enjoyed about playing Apex Predators. First, overwhelming my opponent with big creatures that have trample, and combat tricks like Giant Growth. Second, I enjoyed the "forrests matter" theme. So with that in mind, that's how I built Pack Instinct. There are multiple, valid paths to take with this deck, and I'll be getting into that later.
General Strategy: Overwhelm your opponent's with big, trampling creatures. A turn 2 Garruk's Companion with a turn 3 Leatherback Baloth can be a very powerful early play. The deck just get's scarier as the game goes on, with larger creatures and trample. Prey Upon and the tracker are great for eliminating flyers or other pesky creatures (Soul Warden, Serra Ascendent, flyers, etc) before they can become too much of a hassle. Not much else to say about the general strategy, so let's get into some card analysis.
Primal Bellow - I'm a huge fan of this card. By the time you can play it, it should be at least worth a Giant Growth, and it will only get better from there. I also love it when my opponent goes for a double/triple block, thinking they'll take out my creature, only to meet Primal Bellow. I wish there were more of these.
Garruk's Companion - 3/2 with trample for 2 mana. Yes please, I'll take 4. I've seen some people replace some or all of these and focus more on the ramping cards, but I've always enjoyed having the companion out on turn 2. Start putting pressure on them early, and that just makes your later creatures that much more deadly. In the late game, this goes nicely with Packleader, giving you a (still decent) creature and drawing you a card for 2 mana.
Lightning Greaves - What's scarier than a large creature with trample? Giving it shoud and haste.
Nature's Lore/Cultivate - mana ramp. Also benefits the "land matters" cards, and there's a good number of those. Also, land thinning is good.
Emerald Medallion - Figured I'd talk about this now, while we're with the other mana rampers. There's a few reasons I excluded the medallions this time. They don't help with Companion, Leatherback Baloth, Primalcrux, or Khalni Hydra. They don't help the forest themed cards like Timbermaw Larva, Primal Bellow, Rampaging Baloths, or the upcomming expansion cards. It can also interfere with Indrik Stomphowler (so can Lightning Greaves, but they're worth it). And finally, this deck still has somewhat of a curve, and I don't feel that 8 mana ramp cards are necessary. Every ramp card you have means there's one less threat your opponent needs to worry about removing. This deck doesn't want to be without threats on the board, and I feel that having too much mana ramp would do just that.
Primodial Hydra - Can come down early and single handedly swing for the win after a few turns of its counters doubling and gaining trample. This deck has so many "answer me now or lose" threats with trample, and this is one of them.
Leatherback Baloth - It doesn't have trample, but 3 mana for a 4/5 is just too good to pass up. The 5 toughness also comes in handy agaisnt burn decks, where most of their burn hits creatures for 4 damage.
Chameleon Collosus - It's another big, "answer me now" creature. Protection from black is also very nice, and it can boost its own power. What's not to like?
Master of the Wild Hunt - Yet again, another creature that can get out of hand if your opponent doesn't have an answer. It can also be used to shoot down small flyers with the wolf tokens, or build up a nice army.
Timbermaw Larva - It's biggest downside is not having trample. However, it can put game-winning pressure on your opponent very quickly. A 2/2 for 4 mana is fragile against burn though. It can also combo nicely with Prey Upon in post-combat situations. Overall, not a bad card, but not the greatest. I'll be removing them when the expansion comes.
Garruk's Packleader - 4/4 for 5 mana is kinda meh compared to the rest of the stuff. But that's ok, because the 5 mana creatures here are more for utility purposes, and 4/4 is still a nice body compared to most of the creatures offered by other decks. What's great about the Packleader is that he gives almost all your creatures the bonus of drawing you a card upon entry. This is another threat that needs to be answered by your opponent, otherwise you will quickly overwhelm them with more threats than they have removal for. Overall, a nice draw engine, even if it is a little pricey.
Indrik Stomphowler - I've seen some people remove this card, which confuses me a bit. One of the biggest complaints about Apex Predators was the lack of artifact/enchantment removal. I was hoping to see this card in 2013, and was excited when it happened. There's only one other card in my build that it could interfere with. Meanwhile, there's a number of threatening enchantments and artifacts in most of the other decks. And then you still get a decent 4/4 body with it. I've only found room for 2, but I feel that 3 is a little overkill on the artifact/enchantment removal, given how many good, large trampling threats this deck has.
Primalcrux - Speaking of good, large trampling threats, Primalcrux will be at a MINIMUM of 6/6 when you play it, and it will only get bigger from there.
Primeval Titan - 6/6 with Trample and land thinning for 6 mana. Helps with the forrests matter theme, and gives you more mana to work with for your Packleader draw engine. A solid card.
Rampaging Baloths - Another card that can get out of hand. Try to play it when you have 6 mana available, then drop a land right away, giving you 10 points of power spread across 2 bodies on the turn you play it.
Terra Stomper - 6 mana for an 8/8 with trample. It can't be countered, which is relevant against the, what, 6 counterspells in the game. Still a good card with a good cost.
Khalni Hydra - It'll usually cost anywhere between 6 and 8 mana to cast, which is fair. Goes really great with Primalcrux. Overall, I find him a little expensive for what he does, and thus got cut with the promocards.
Fog - I never cared for Fog in a green beatdown deck. To me, I should be going on the offense, not stalling for a turn to draw 1 more card that I normally would have drawn the turn before had I not had fog in the deck. Some people like it, but I don't.
Blanchwood Armor - There's a few reasons I excluded this card as well. First, it's not a combat trick. Second, it can give opponent's opportunity to gain a 2-for-1 card advantage. I would rather just play another trampling threat than to just power up something that doesn't really need more power. And third, the large trampling creatures generally don't need more power in this deck. This deck already has some of the biggest creatures in that game.
Fangren Fistborn - Good for the token build, but I'd rather be dropping a timbermaw larva, which will offer more immediate pressure.
Jayemdae Tomb - Really slow draw engine. It takes a total of 8 mana just to draw your first card off of it, and that card would only go to replace the tomb. That means it would take a total of 12 mana to start getting card advantage from it. No thanks, for that price, I'd rather be dropping threats that kill my opponent. Packleader is a good enough draw engine for this deck.
Primal Huntbeast - Blanchwood armor would go better if I was running this. It's still lacking trample though, hence I cut both. Hexproof is nice, but the huntbeast just isn't big enough imo.
Ant Queen - Good for the token build, but I usually find myself not making too many tokens before either the game is over or the queen gets hit with removal. My strategy isn't tokens, it's trample, and for 5 mana I usually find myself wanting to play one of the other 5 mana utility creatures first. When a card is the last card you want to play for that mana cost, that's a good indication to me of a card to cut. I'm not saying it's a bad card, it's certainly good for the token path, but my build just isn't that.
Predatory Rampage - This would be better if it granted trample (coughoverruncough), but it's a great booster for a token build. It could also be used as a mass Prey Upon, although, that seems a bit situational, at least for the non-token build.
Sentinel Spider - This deck does have issues with flying creatures. However, Prey Upon and Tracker can help remove fliers, and this deck is already filled with a lot of good stuff at 5+ mana.
Epic Proportions - This card has flash and grants trample, which solves two of my main complaints about Blanchwood Armor. However, which this can be used as a combat trick, it's a 6 mana combat trick. For 6 mana, I'd rather drop another tampling threat that would be larger than 5/5 instead of opening myself up to giving my opponent card advantage by playing auras.
Boundless Realms - I've seen some people include this, which is the only reason I'm commenting on it. A seven mana spell should be something game winning. This only puts more basic land into play, and it puts them into play tapped, so there goes your whole turn. By the time you have seven lands, you need to be dropping threats and game-winning bombs, not getting more mana. On the positive side, it will thin most of the lands out of your deck. If the lands were to come into play untapped, it could warrent use, as it would be great with the forrests matter cards.
Howl of the Night Pack - Hey look, an instant army. I like this card, I really do, and it's perfectly valid to include, token build or not. This can help you recover from a board wipe, stop an opposing ground army in its tracks, and provide you a game winning attack. It doesn't trigger Packleader and they aren't large trampling creatures, but those are relatively minor points that really don't hinder the quality of Howl. However, on the negative side, it does cost 7 mana, so seeing it in your starting hand can hurt, even with ramp. 6 mana cards hurt to see early on too, but those just feel more... managable to see earlier.