For the longest time I have played Hecarim as a tank/fighter with a trinity force and spirit of the elder lizard as my main damage sources. Recently I discovered that despite his low AD scaling, his utility and passive make for some surprise burst damage. The best things about AD Hecarim are that your Q doesn't interrupt your basic attacks and your E applies your sheen proc which allows for crazy burst damage. With a decent amount of armor penetration and cooldown reduction, AD Hecarim can melt a carry in seconds and sustain through their damage with his W active. Late game with all speed buffs I typically have over 500 AD, which is why an infinity edge is viable.

Obviously you wouldn't want to be super squishy if your team is already squishy and has no fighter/tank.

I'll play with this build and nearly always do very well if our team has some sort of fighter or tank with an initiate such as jax, warwick, leona, etc.

The most important thing about being a jungler is map awareness. Pay attention to when someone gets caught out and you can most likely save them due to your extremely high movement speed with E, ghost, and youmuu's active. A good save tactic that I have used several times to make people sh*t bricks is to ult through a wall to fear them, give them a little push with your E + Q, and then safely run away with youmuu's and ghost if you have it.

If you want to gank, GANK HARD. An easy way to do well as AD Hecarim is gank someone early that has no escape except for a flash. Do your best to charge in with E and your W active (and even ghost if you need the extra damage and element of surprise) and push them towards your turret while spamming Q, and if they run (which they should) run ahead of them using your Q and your W to consistently damage them without stalling for any basic attacks unless you can afford it (similar to how you would play phoenix udyr). It is crucial to save your ult for the fear. If they flash and you were running ahead of them, you can easily ult in front of them and fear them back towards your teammate who can secure the kill. Saving your ult is especially great when going against someone like a Master Yi with his ult active so that your fear makes him run super far away because he can't be slowed (it's pretty funny to watch).

Late game, focus the squishy champions as always, and don't be scared to 1v1 if someone is caught out. When trying to 1v1 someone that can most likely burst you down very quickly (particularly AP carries) you should try to surprise them by activating your E, W, youmuu's ghostblade, ghost, and Ultimate in quick succession. While they are feared from your ultimate, land your E on them and use your Q as soon as possible. At this point they should be dead (: