Hi, welcome to my Jungle Hecarim guide (and my first ever guide)!

First off, if you just randomly chose Hecarim and need to see what skills to level and what items to buy, see the information above and come back to read the guide later.

So, why play Hecarim?

In the Season 5 Meta, we're seeing a LOT of lane bullies top. Ranged champions, fighters, and even Teemos. Tanks struggle against these champions, so by playing a tank jungler you give your top laner the opportunity to play a lane bully and have a better laning phase while still having a tank on the team. On top of that, Hecarim has AMAZING gank potential. He can sustain well in the jungle and get all 3 lanes snowballing.

Pros

Cons

Good ganks pre-6, Great ganks post-6

No gap-escape aside from his ult

Strong Split-pushing

Weak 1v1 early, susceptible to invades

Amazing teamfight presence

Very Blue Dependent

Strong sustain in jungle clears, great objective control

Needs damage from team to get sufficient heal from W in teamfights

Hecarim has a pretty standard Rune page for a Jungler. Nothing should be surprising here.

Marks - Armor Pen
Seals - Flat Armor
Glyphs - MR / level
Quints - Movement Speed

Other options:
Don't even consider changing marks or seals, these are absolutely necessary to do the early jungle. AD marks work, but not as well as armor penetration.
Glyphs - CDR will work, but MR/lvl will do better unless the other team lacks AP or you shut down their AP champions hard.
Quints - Movement Speed is so core on Hecarim that I'd strongly suggest taking these. Other options would be Attack Speed, Attack Damage, Armor Penetration, or Armor if you need to be really tanky.

Items are pretty self-explanatory.
Stalker's Blade - Warrior: Stalker's blade is a no-brainer. You now STEAL 20% movement speed. So your ganks are better, teammates can chase the slowed opponent, and you get bonus damage from your movement speed increase! Warrior enchantment provides early damage and armor penetration to help with faster jungle clears and more damage on ganks. Don't fall in to the trap of buying Ranger's Trailblazer, Hecarim has no problems clearing the jungle with his AOE skillset.

Iceborn Gauntlet - Sheen is ridiculously core on Hecarim because his Q has a base cooldown of 2 seconds after 2 stacks. Your options are Trinity Force or Iceborn Gauntlet. Trinity Force IS BETTER, but IS TOO EXPENSIVE for a jungler. The movement speed and stats on TF are definitely better with Hecarim, but you'll complete Iceborn Gauntlet much faster and the AOE slow+damage on hit after spellcast will help your ganks and AOE damage even more.

Spirit Visage - Passive goes with W, enough said. Provides health and magic resist which works great if the enemy has AP.

Randuin's Omen - Health and armor, and an attack speed slow.

Sunfire Aegis - Health, armor, and even more AOE damage to go with his kit.

Boots:
Choose between Mercury's Treads against AP heavy teams or teams with multiple (non-knock-up) forms of CC. Go Ninja Tabi against AD heavy teams that rely on auto attacks. Go Boots of Swiftness if against a team with multiple slow abilities.

Alternative Options:
Frozen Heart for 20% CDR and 100 armor works wonders against AD heavy teams. Rush this after Iceborn Gauntlet in these cases to hit the 40% CDR cap after 3 items.
Thornmail for a cheap item when against a fed ADC.
Banshee's Veil against poke comps and for extra magic resist.
Ohmwrecker and Zz'Rot Portal are interesting items worth trying if you get the chance. The 5.5 point runner buff may make these exceptional snowball items after you complete your jungle item, sheen/iceborn guantlet, and boots.

With the above runes and masteries, starting W at gromp with a leash will allow you to clear the entire Blue side of the jungle on 1 Health pot (see the Jungling section for detailed information). **As of 5.2, you need more gold to upgrade your machete so you can't base with that 1 extra health pot anymore - regardless starting W is optimal, you now just drink the health pot and go do Red then Raptors.

Level R whenever possible. The base damage increase is HUGE and the base cooldown reduction is incredible as well.
Max Q first, this is your primary source of damage for clearing camps and ganks.
Choosing between W and E for second is situational.

I almost always used to max W second because I look to have a large presence in teamfights and in my games people seem to start midgame pretty early so teamfights occur more often. You can go E second if you're looking to Split push or if people are staying in lane and you can pull off ganks more often. The 2 second base cooldown reduction per level on E is a huge improvement. I've started to prefer maxing E over W as W's healing is uncapped against champions in teamfights (so you're only improving the damage which isn't huge) and the mobility of your E helps you get to teamfights faster and lets you peel with the knockback more than once per fight.

Ghost - Way better than Flash due to his passive. Lower cooldown too.
Smite - You're jungling, duh!

Other options
Teleport - Hecarim top has a 53% win rate for a reason, he's one of the best in the game at teleporting in to fights. Throw a homeguard enchantment on an early boots upgrade and you can secure a teleport kill pretty easily.

In lane though, you can consider
Teleport or Ignite

Your jungle path will start the same every game.

Start with Hunter's Machete and two Health Potion. Run to the river on the side of your blue buff IMMEDIATELY and use your trinket to place a ward. If an invade appears to be incoming, ping your teammates on the opposite side of the map to rotate for help. Otherwise, ask your red side lane to ward your red buff around 1:45.

Start Gromp. If you're playing Red team, your support and ADC should provide 2-3 auto attacks each for a leash. If you're Blue team, your Top should provide 3-4 auto attacks as a leash. Either way, tap W as soon as you take your first damage from Gromp, and use Smite after taking damage a second time to maximize your healing from W.

After killing Gromp, you'll hit 2. Level Q, and go to WOLVES. Attack the big wolf, and tap Q immediately after your auto attack. Drink your first health potion. Tap Q as soon as it's off cooldown. You now have 2 Q stacks, as soon as Q is about to come off of cooldown activate W then Q again. This will maximize your damage while W is active thereby maximizing your healing. Keep hitting Q whenever off of cooldown and you should kill all three wolves at the same time with your last Q. Immediately run to Blue buff and use Q BEFORE THE STACKS WEAR OFF. You'll keep up both stacks of Q and it will stay at a 2 second cooldown. Spam Q on the blue buff until W is about to come off of cooldown, then activate W and follow with Q immediately thereafter and spam until the camp is cleared. Drink your second health potion during this camp. This camp gets you level 3, take Q again. Run to Red Buff, throw a ward over the wall behind it, and attack red. Run to the brush beside Red while fighting red, remember to get 2 Q stacks before using W and also use smite while W is active (you'll also heal a great deal from Smiting Red). Next, run straight to Raptors and clear them as well, making sure to keep 2 stacks of Q up between camps. After 5.5 changes weakening Gromp and Raptor's damage (also weakens Gromp buff on first clear), you can actually proceed to clear Krugs with another smite to hit level 4 (so you have e after recalling to get back to the jungle faster) and you'll have more money.

* Note: You start Gromp because Hecarim is a tank jungler, and go blue because Hecarim is VERY blue dependent! Doing Blue third (you'll have only a little mana left after clearing blue) maximizes your mana recovery out of the blue buff.

If you're still level 3: After recalling, you can buy your Stalker's Blade and 3-4 more health potions (can get 1 mana potion, and I'd probably buy a pink ward). Go Krugs or Wolves without smite to hit 4 and take your E, then look for your first gank.
If you're level 4 (did a full clear): You can buy your Stalker's Blade, a Longsword and 1-2 health potions. You forego early vision from the pink and some early sustain if you take damage in ganks, but gain 10AD from the Longsword which more than makes up for it via easier camp clears and better ganks. Boots would also be an option with an extra health/mana potion (his passive will grant AD from the MS), but I prefer Longsword as it builds in to the warrior enchantment and lets you complete it more easily.

From this point onwards, clear the jungle when your health is too low to go for ganks, when buffs are up, or when no ganks are available. Prioritize:
Blue Buff > Red Buff > Raptors > Wolves > Gromp > Krugs.
You can smite Gromp for the buff and while your W is active to increase your healing for an easy camp clear. Otherwise, camps with more monsters are easier to clear since you're AOE and obviously you need to keep your buffs away from your opponent. Don't be afraid to take a mana potion and give your second Blue buff and onwards to your mid laner. Even without mana potions you should be okay to give Blue to your mid after you have mana from Sheen.

Stealing Objectives

I'll keep this short and simple. If the enemy team is taking Baron or Dragon and you can't contest it with a direct fight, Hecarim has an option. Follow these steps:
1. Use a ward to establish vision
2. When the objective approaches smite-able health, use Onslaught of Shadows to enter the pit. Ideally land the fear on as many opponents as necessary.
3. Smite-kill the objective.
4. Activate ghost and E and try to run out as fast as possible (or if your team finally showed up, fight!).

Ganks

Pre-6

Gank using E and your chilling smite. Wait for the lane to push and target ping your target so your laners know to go in. Activate E to start the MS speed boost (it ramps up from 25% to 75%) and run in behind your target and hit them with Chilling smite to steal MS (and amplify your damage via the passive). Push the target towards your laner using your E knockback, activate W and spam Q while staying on top of them and auto attacking when possible.

Post - 6

Much the same as pre-6, except you now have Onslaught of Shadows to improve your ganks. Typically save your ultimate until they use any dashing abilities to escape from your E/Smite. Then ult on to them, aiming to land the fear radius just past them to fear them back towards your laner.

Teamfights

Hecarim has a strong engage. Activate E to run in (you can get MS boosts from teammates, smite a target to steal speed, or use Ghost). Initiate with your ultimate to land the fear on their ADC, APC, or ideally both. Use the E knockup to further push their ADC towards your team. Focus down their carries, and activate your W as soon as you start taking serious damage.
W only has capped damage against minions and monsters, it is uncapped on champions. Make sure your team knows to follow up with damage, as long as the enemy team stays within your W radius the healing you receive from your team's damage will make you INCREDIBLY tanky. This deceptive tankiness will often make the enemy team focus you while you receive incredible healing over the 4 seconds of your W.

As always, spam Q and when your W runs out and you get close to dying, use your E and other MS abilities to escape the fight when you can't peel for your teammates anymore. Remember, a good tank dies to save their carries, a great tank saves their carries, wins the fight, and gets out ALIVE.

Hecarim combines all the most needed elements of a jungler:
CC and Initiate power for early game ganks and late game teamfight initiation.
Huge MS boosts for split pushing objectives late.
Sustain via W for jungle sustain in the tougher jungle.
AOE damage for teamfight presence and fast clears.
The ability to customize your build to enemy teams, and to have an expensive and damage-oriented build when you're getting fed and a cheaper and defense-oriented build when you give gank-kills to your team and need to be the tank-initiator on the team.

So, I hope you all found this helpful. If you did, please up-vote! In any case, please comment with constructive criticism and feel free to give me any advice considering this is the first guide I've ever written.