I do not own Hecarim. I played him as a free champ and he was good but not my style. But I noticed most guides for him publishing 4+ builds and getting upvoted for getting one right and I hate that. "Spam them all! One is bound to work." example. So I thought with a champ whose passive turns movement into AD there should be a build that maximizes AD and Movement.(Note: the example is also the currently highest voted Hecarim guide so if you don't like mine go check that one out)

Pros +Crazy Damage +Insane Movement +An Ult that can go through walls +Looking Awesome

SPACE

Cons -Weak Early game -Needs good teamwork -CC makes him cry -All the pony jokes

Greater Mark of Desolation

Basic mark for any AD build. You could replace it easily with Greater Mark of Attack Damage or Greater Mark of Attack Speed or even Greater Mark of Critical Damage if you want.

Greater Seal of Armor

Generic seal to use. Greater Seal of Health, Greater Seal of Endurance, and Greater Seal of Regeneration/ Greater Seal of Vigor are all acceptable replacements. Just use whatever you find works best.

Greater Glyph of Magic Resist/ Greater Glyph of Scaling Magic Resist

Since you want to use all Primary runes magic resist are really the only viable Glyphs for an AD build.

Greater Quintessence of Movement Speed

No brainer considering this build is about maximizing move speed.

Offensive

Summoner's Wrath : This is just to power up your summoner's spells since both of them fall under this ability and it gives you more move speed with Ghost which is what this build is all about.

Defensive

Really the only thing in this tree that's important is Initiator and the tree only needs 15 points to max that out I just felt using the other 6 to get Juggernaut was the best use of the remaining mastery points.

Utility

Once again you're really aiming for 1 mastery in this tree Swiftness and once again it's all about maxing out your move speed.

Berserker's Greaves

Basic AD boots. Can be replaced with Boots of Swiftness or Mobility Boots

Bloodthirster

Life steal is very important to your sustain and so I suggest getting at least Vampiric Scepter early in the game if not Bloodthirster to help on that front. If you plan on jungling replace this with Wriggle's Lantern.

Phantom Dancerx2

Here's where most of your movement is coming from. These give you 15% speed boost as well as critical damage and a huge boost to your attack speed.

Trinity Force

Just a good all around item that initially had been a third Phantom Dancer but I replaced it after realizing that a 3rd one gives me wasted critical strike chance whereas with Trinity Force the critical chance is exactly 100%.

Infinity Edge

With this wrapping up the build it puts the critical chance at exactly 100% and with its passive boosting the critical damage even higher and the bonus AD you're gonna be hitting for 2.5x your reported AD. Throw your Trinity Force boost on top of that and you're mowing through your enemy's HP like it's nothing.

Rampage

You wanna max this out first due to it being your only real straight damage skill. It can be combined with your Devastating Charge to inflict extra damage or with the Onslaught of Shadows for huge area burst damage.

Spirit of Dread

Important for jungling not so much in laning. Still a good skill to have that helps with sustain and does a little bit of damage but in a fight the heal/damage can save you or finish your enemy.

Devastating Charge

A very flexible skill you can use it to gank, get out of trouble, inflict just that little bit more of extra damage to an enemy, run back to position after returning to base, it can be used in tons of different situations. And the damage it does is based on how far you've traveled while it is active meaning higher move speed = higher distance in same time = MORE DAMAGE. Not to mention it boosts move speed itself.

Onslaught of Shadows

Good ultimate that has just as many uses as Devastating Charge the main difference being this can travel THROUGH WALLS! making it great for a surprise Hecarim gank. Can also turn a team fight around with the wide area of effect and fear.

Ghost

Once again all about the move speed. Plus other obvious uses of Ghost like getting to different capture points in dominion or chasing down a running champion or running away.

Ignite

Good summoner skill for an aggressive build like this. Can also help a lot against annoying heal supports like Soraka and Taric.