Hecarim in my opinion is a great jungler, with high mobility, damage, and a few other things. This guide isn't 100% accurate, but I use roughly the same thing every game and it works out pretty well. But don't be afraid to change it or tailor it to your play style. I just find these items and ideas work well in conjuncture with one another and Hecarim. I use these on a regular basis and it works out pretty well.

Pros

High mobility and map presence
Good damage
Clears jungle quickly
Can gank early.


Cons

Not as tanky as some junglers.
Large model prone to skill shots.
E and ult does more harm then good when used wrong generally

I chose 11/10/9 masteries for simple reasons. The 9 points in utility give you the rune buff, and a nice little movement speed boost too. That's the main reason why. The 10 points in defense get you through the jungle. Plain and simple. Then the 11 points in offense: the 2 in butcher are there to get you through the jungle and the rest are there to help you deal damage to champions.

I chose the movement speed quints to get me from point A to point B as fast as possible. The armor seals to get you through the jungle, the scaling magic glyphs to get you survivability late game, and the flat armor pen runes to deal some extra damage.

Your Q is your primary source of damage other then basic attacking. Every time an enemy's in range and you can use it, use it. It costs so little mana to the point where it's barely even noticeable.

Your W is your sustainability. Use it when the fight starts and you've taken a little damage so that way it can get back on cool down later on in the fight.

Your E is your pre-fight move. Use it a bit before you engage so you can position your opponent by knocking them away. It'll get some practice before you get the timing down so you hit them right at the end of it to maximize damage. Also, if possible, use your Q before performing a basic attack while in this to truly maximize damage. Also, if you're on Skype/vent/teamspeak/have some form of communication, sing About your horse whenever using this move, and for a bit after.

Your ult is generally how you come into larger fights. Use it to scatter an enemy team and follow up with your E against a low armor or high priority target to take them out after separating them from their team. In desperate situations it can be used as a get away move, but it's better to use your E in that instance. It also charges through walls. Don't forget that you don't instantly arrive at your destination, and that you need to predict where they'll be.

So, cloth armor and five red potions right from the get go. Gets you through the jungle, though you sell the cloth armor later on in the game. Then follow up with a boots of speed to get you around the map and keep up with ganks. Sheen after that as it spikes your damage up, as your Q resets your attack timer and is on a low cooldown. Grab Boots of swiftness to get you around faster and deal more damage. Then finish up your Trinity force, follow up with an infinity edge, if you need more survivability, grab a force of nature (unless you're against an AD heavy team, in which you build something with armor like Thornmail), then your infinity edge. If you are doing well and don't need more survivability, then build your infinity edge, a zeal (don't finish it yet) and then your force of nature. Build your bloodthirster for life steal and then top it off by turning your zeal into a phantom dancer. Sell your cloth armor whenever you need the extra gold to secure a purchase on a larger item like zeal or something.

That image sums up your early game route pretty well I think. Go B after you take out wraith's the second time, grab some boots, and gank which ever lane you feel you have the best chance of securing a kill in. Then be sure to not spend too much time ganking but not too much time in the jungle, and don't be afraid to dunk the hell out of someone. If a squishier enemy's below ~30-40% health dunk them! The tower will deal a chunk of damage but like Nasus says: "We will leave, they will die." Just don't be stupid.

How to tell which lanes are best to gank:

If they don't have a solo topper gank top.
If mid doesn't have hard CC and doesn't have a way to escape gank mid.
Gank bot if it's pushed.

Use your E and ult to push them towards your team, attack low armored targets, and use your high mobility to your advantage. Never be afraid to disengage or poke. Hard CC and slows will screw you up. You can also deal a lot of damage to turrets, so don't be afraid to push lanes when you can.

If you have any questions or comments or would like me to expand on anything just let me know. I'll do my best to answer them.