Hello everyone, this is my personal build for my favorite jungler Hecarim. I like to play him as a bruiser that starts the game with high health, adding damage throughout the game while still improving his defense.

Greater Quintessence of Swiftness - These are just a logical choice for Hecarim, more MS works very well for him, allowing him to stick to targets with high mobility and adding damage from his passive.

Greater Mark of Strength - These will help your clear times a lot in the early game, which is absolutely necessary since your ganks are not very strong until you get your E.

Greater Seal of Resilience - These are a no brainer, more armor for a jungler means more health for ganks, longer time in jungle.

Greater Glyph of Shielding - Again, a no brainer, the best glyphs to get for almost every single champion.

Other good choices

Greater Mark of Desolation, Greater Quintessence of Desolation - This is something I haven't messed around with a lot, haven't done any calculations to see if these will prove to be viable for Hecarim. If someone wouldn't mind doing that for me and put it in the comments, it would be greatly appreciated.

Greater Quintessence of Strength - An excellent choice if you don't have the Swiftness ones and don't feel like blowing 6k IP on them.

I run a 0/21/9 for a few reasons. I like to get very tanky during the early game, making me nearly impossible to kill when combined with my high mobility. That is another reason why I use this setup is to get all the MS masteries. Also, as a jungler, the Runic Affinity is very handy for keeping your buffs up as much as possible.

Starting Items: Boots of Speed and 3 Health Potions
Great movement speed to traverse the jungle quickly so you can get to level 4 as quickly as possible. This will also boost your damage through your passive.

First Buy: Ruby Crystal
Get this as soon as you can because of your lack of HP. You will later build this into a Heart of Gold

Early Core Items: Merc Treads, Heart of Gold, Phage
These three items will give you high HP in the early game and a very useful passive if it procs

From here, follow the build I have provided above. Try to keep your heart of gold as long as possible to maximize the gold gained and increasing the damage gained via Atma's. Remember, these items are for an average game where they have a balance of AD and AP damage.

If they have high AP damage, switch your Atma's for a Force of Nature
If they have high AD damage, switch your Maw for a Frozen Heart
Your damage will suffer, but if you build either of these items in these situations, you will easily be able to outlast them and kill them before they kill you.

Rampage - Your main ability for good clear times and very useful for chasing enemies because you don't stop moving when you cast it, allowing you to stay right on top of them and chip them down. You need to max this first for the faster clear times.

Spirit of Dread - This ability is absolutely amazing with a good team. During the jungle, it gives a minor amount of AoE damage and decent sustain, especially if you use smite while you have it up. This ability really shines during teamfights if your team does good damage. As long as your team dishes out high damage and you stick close to where the damage is, you will not die. Max this second for the increased sustain.

Devastating Charge - Your ganking ability. Time this correctly so that when you reveal yourself from the brush, you will have built up a large amount of MS. You can do this by activating this ability half a second or a second before you charge, depending on where your victim is in the lane. Try your best to position yoursel behind your target so you can knock them into your teammate. If you can't, knock them towards the wall. Use this at the same times as Ghost will almost always ensure an easy gank. This ability also works very well with your ultimate, which I will explain in the ultimates description. Max this last because the utility is the most important part of it.

Onslaught of Shadows - An extremely underestimated ability, great for positioning your E and potentially fearing the entire enemy team. Use this to initiate a teamfight by jumping on top of their ADC. Before you do this, activate your E and when your ult finishes, you should be behind their ADC, which allows an easy knockback into your team, throwing their positioning off which should result in an easy kill. Max this as much as possible for lower cooldowns and increased damage.

Smite - Hecarim is a jungler, take smite.
Ghost - Extra MS is exactly what Hecarim wants, the more the better. The only downside to this ability is the waste of no unit collision since you get that from your passive, but it is still worth it.

Other options:
Flash - Great choice to allow you to position yourself better for your E.
Exhaust - A good skill to pick up if you feel like you don't need the higher mobility, personal preference really.

To wrap it all up, with this build you can play Hecarim as a very mobile bruiser great for initiating teamfights and being a major pain for the enemy squishies. Hopefully this guide helps, any tips would be greatly appreciated.