Hecarim is a strong jungler who is underplayed in my oppinion. His ganks are very strong, even pre 6, if he lands his Devastating Charge and become even stronger at 6 when he can even further misplace the enemy laner using the fear from his Onslaught of Shadows.

His clear speed is also strong, although slower than the likes of Shyvana and Dr. Mundo, however this is offset by his more reliable ganks in most cases. As for sustain in the jungle he deals decently well as long as he has a good leash on blue, and should finish his first clear with somewhere between half and 1/4 health presuming he clears fully with no ganks.

Pros

• Strong jungling speed with no investment into damage due to AoE clear of Rampage in combination with Spirit of Dread.
• Snowballs very hard due to the nature of the way his abilities combo with Trinity Force.
• Strong AoE area control with his AoE damage and cc through Onslaught of Shadows and Devastating Charge.
• His passive Warpath provides more damage as you aquire more movement speed, which is a desirable stat anyway, so a nice little boost however not worth building around in its own right.

Cons

• Vulnerable to counterjungling at early levels due to lots of his damage coming from abilities, which are weak damage wise at very early levels (1-6ish).
• Rampage takes about 6 seconds to stack up to its full effectiveness which leads to a wind-up time for his ganks, similar to Skarner.
• Newer Hecarim players will often misuse his Devastating Charge, resulting in the enemy being knocked away from your teammates rather than towards them.

For runes:

• Greater Mark of Attack Speed allow for a faster clearing speed at lower levels allowing you to gank more easily. Flat ad marks can also be used to a similar effect, however I believe alacrity marks are more effective due to the fact that Hecarim's autoattacks are not interrupted by his Rampage animation.
  
• Greater Seal of Armor are practically a necessity on all junglers.
  
• Greater Glyph of Scaling Magic Resist or Greater Glyph of Magic Resist are both viable options, scaling magic resist will provide more magic resist after level 9 so I believe they are the more effective choice.
  
• Greater Quintessence of Movement Speed are optimal on Hecarim and with a boots start will provide the same amount of ad as flat ad quints would due to his passive, Greater Quintessence of Attack Damage or greater quintessence of desolation can be used if you do not have movespeed available.

I prefer 0/21/9 masteries on Hecarim as I believe the offense tree provides very little when compared to the movement speed bonuses available in the utility and defense trees, which I think are essential on hecarim due to the double-effectiveness provided by his passive.

The Indomitable and Tough Skin / Bladed Armor masteries are also essential on Hecarim, as they are with most junglers, due to the massively reduced minion damage at lower levels making the first jungle clear that much more easy.

Durability / Veteran's Scars also help towards this cause.

Any other mastery choices I believe to be non-essential and mostly down to choice however I like to take Expanded Mind in utility for some extra mana, which you will be using for his Rampage, and Improved Recall as it does save some time when a recall is needed.

Runic Affinity also helps towards making you more effective in the jungle during your first clear due to the increase duration of blue buff allowing for more AoE damage with Rampage.

I recommend going for a full clear on Hecarim, as his early ganks are relatively weak anyway and he need to be at least level 3 in order to do anything useful during a gank.

As such I start at wolves clearing them as they spawn at 1:40 with the help of my team if available, then move on to blue at 1:55 with a leash from my mid laner so that i take less damage at the beginning and in the jungle overall.

I then move on to wraiths, followed by red and then small golems. At this point you can choose to gank early (as you should be around half health) or continue to farm to ensure you can get a philo stone on your first recall. This is done by going back to wraiths, then wolves then finally small golems again, which should allow you enough gold to purchase a philo, or at least put you very close to the 800 gold you need.

As for actual technique, you should be using both your Rampage and Spirit of Dread on cooldown as Rampage has almost no cooldown and costs very little mana, so it is basically free damage. Spirit of Dread also has a relatively low cooldown and is simply an activateble AoE which provides sustain based on how much damage you, and nearby teammates are doing, so it should be used whenever possible in the early levels.

The items for Hecarim are fairly standard for all tanky junglers

• I prefer a Boots start on almost all junglers due to less money being wasted on Health Potions than would be with the Cloth Armor alternative, also almost every strong jungler can clear the jungle these days without needing the 5 Health Potion or the extra armor from a Cloth Armor.
  
• Double gp10 philosopher's stone Heart of Gold has become a staple of most junglers at this point due to its low cost and constant income throughout the game.
  
• Sheen for a massive increase in early game damage for a low cost.
  
• Glacial Shroud a strong item in its own right and the precursor to your Frozen Heart.
  
• Mercury's Treads are a must purchase on near enough every tanky character due to the cc reduction meaning more damage over time, as you do 0 damage while you are cced and the other boots options provide little for offense. Mobility Boots are a possible alternative if you wish to focus on very heavy map control and ganking. However I would not recommend this outside of a fixed team. Boots can be upgraded earlier in the game if you feel it is necessary.
  
• Frozen Heart is an amazing item which provides tonnes of useful stats for a relatively low cost, the cdr combined with the mana allows you to spam Rampage more often and for longer, resulting in more damage, while the attackspeed reduction for enemies combined with the 99 armor make you very resistant to physical damage, and all that for just 2775 gold. With this item early, around 18-20 minutes if you are doing well, you will be near enough untouchable by the enemy ad carry and deal tonnes of damage with repeated Rampages, which brings me on to my next item.
  
• Trinity Force, I actually do not view this item too highly on most champions, however i believe it is strong on hecarim as he makes use of absolutely all of the stats, which could partly justify the ridiculous cost of 4070, if you feel that you do not need to do more damage this item can be skipped. I do however recommend this item as it synergises very well with the repeated spamming of Rampage resulting in repeated procs.
  If I am building this item i prefer to build Sheen first as at that point in the game (~10 minutes) I will have red buff most of the time and am not in need of the Phage slow. However Phage does provide more survivability than Sheen, so if that is what you require feel free to pick up Phage before Sheen.
  
• Shurelya's Battlesong, this item is extremely strong as it provides a ghost to your entire team for a short time and is a cheap natural progression from the philo stone which you picked up earlier.
  
• Randuin's Omen is built for a similar reason, it is very cost effective providing a mild AoE slow and damage reduction which scales with armor (which you will have a lot of at this point).
  
• Finish the build with either Guardian Angel (if you are carrying your team and are essential to teamfight victories) or Force of Nature (if you are looking for more resistance to magic damage, or if the enemy ap carry is particularly fed this can be built earlier instead of Randuin's Omen or before Shurelya's Battlesong).
  
• Maw of Malmortius is also a strong item on Hecarim when the enemy team is heavy on magic damage and can be built. I would probably build this instead of Shurelya's Battlesong if it was needed. If you do prefer to build this item and are unlikely to ever build Shurelya's Battlesong then I would not recommend picking up the early philosopher's stone and would instead skip straight to the Heart of Gold.
  

A final note: I do not favour Wriggle's Lantern on Hecarim as I believe the proc does not synergise particularly well with his kit and your W skill will provide all of the sustain you will ever need in the jungle.

TLDR: >>>(philosopher's stone)>>> >>> >>> >>> >>> >>> >>> or >>> >>> or .

Take an early point in Spirit of Dread for the jungle sustain (1st or 2nd point should be spent here). Your second point should be spent in either Rampage or Spirit of Dread depending on which you put your first point into. from this point the skill build depends largely on preference.

I like to get a 2nd point in Rampage at level 3 as I rarely gank before I have finished my initial clear, however if you notice an opportunity Devastating Charge can be ranked at level 3 for an early gank (I would not recommend ganking any earlier than this on Hecarim).

After this point either Rampage or Spirit of Dread can be maxed. I prefer to max Rampage over Spirit of Dread as i believe rank 1 Spirit of Dread provides more than enough jungle sustain while Rampage provides much more damage than Spirit of Dread both for clearing and ganking. I max Spirit of Dread second as the healing per damage % increases as you rank it up and at the higher ranks your teamfight sustain will become ridiculous as long as you play correctly.

I max Devastating Charge last as it proves very little through ranking it up apart from a small reduction in cooldown.

Obviously Onslaught of Shadows should be ranked at 6, 11 and 16 due to the power of ultimate abilities.

• Smite is an obvious choice when you are jungling due to the utility it provides at all stages of the game, early game damage as well as late game buff security.
  
• I take Flash on Hecarim as I like to be able to instantly reposition myself, either to pull off a strong knockback with my e or in order to escape over walls.
  
• Ghost is also a viable alternative as it provides damage (due to Warpath) on Hecarim and is better for chasing than Flash, however it does not provide the same instant utility as Flash, it is a situational choice.
  
• Exhaust some junglers have been running exhaust recently such as Shyvana and Dr. Mundo, however this is to compensate for their lack of reliable cc, I do not believe Hecarim has this problem however so I do not take Exhaust on Hecarim, however it could be a viable choice when played correctly.

• Flash can be used to reposition yourself for a quick Devastating Charge knockback into your team if you are not going to be able to run past the enemy in order to do so.
• Similarly Onslaught of Shadows can be used to run in front of an enemy causing them to run away from you and towards your team, Devastating Charge can then be used for an even further knockback.
• When ganking try to land at least one Rampage on the enemy while Devastating Charge is active, as the extra movement speed provided by it will result in extra damage on your Rampage.
• When in the jungle try to finish clearing the camp with your Rampage in order to preserve the extra cooldown reduction for the next camp, this will result in a slightly faster clear time.

Hecarim is a strong choice if you are looking for a tanky jungler who also provides lots of strong soft cc and strong mobility along with area control. I believe he is undervalued at the moment and may soon become a flavour of the month jungler as soon as some of the pros take notice of him, the only problem is that he does need items in order to function well, which could hold him back in the early gank-heavy professional scene.