Hecarim is a desruption constant damage unit, at least thats how I play him. A mobile offtank. Going for a nice jungle and great ganks. Hecarim is also a great battle starter with his ulti moving everyone out of position and then using his Devastating Charge to shove the support of carry to his team. All in all a very nice and powerful hero to have.

STILL A WORK IN PROGRESS I WOULD LOVE SOME HELP FIRST BUILD GUIDE.

My masteries go 12/9/9 giving damage survivability and utility. In the offesive tree I get the boost to ghost and the 3 base damage to start. This is just standard damage. Next I go for attack speed which is just nice but so I can also get Weapon expertise for the armor pen. Finally taking havoc for nice damage increase.
In the defense tree I get the boost to smite and some tankyness to minions always good for jungle. Then I go for the health for some more survival power.
Finaly in the Utility tree I gett the mana so you can keep up that Rampage longer and the faster recall for whenever I have to return to base. Next is the swiftness to give mobility and it works nicely with his passive. Finaly taking Runic affinity for longer buffs in the jungle.