Hecarim is quite strong if played correctly. If you farm up enough u will get kills and by the end of the game 2 hit ko the carry.

Either start with boots or cloth armor. I pick up more mana then health potions so u can sustain with your w. Next go into a zeal a giants belt and another zeal. this gives you incredible speed and sustain. Build your warmogs merc treads and phantom dancer and if you can a trinity force by late game. if it is one of those games were you have 20 plus kills buy an infinity edge and another phantom dancer. Your basic attacks will do 700+ damage on an average carry.

Pros:
incredibly fast
semi tanky
tons of damage
whoever you target will die

Cons:
CC kills you
Lane can be hard

Early game play safe farming with last hits. At level three you can harass with your E and if necessary heal with your W. Once you get your zeal and giants belt you can be a major assassin.
As teamfights break out ult in and go straight for the carry. Use your W so you dont die and keep after the carry no matter what. Then either go back in if safe or E out. Keep this up and you will get very fed.

Well this is my first guide it is kind of rushed and i will continue to edit it untill perfect. Please dont right of this great champion.