Hecarim is mainly a soloq champ, not seeing any pro play. He's an okay champ against disorganized teams, so if you can take over the game, you'll probably win. The only problem is, you can't contest crabs, and you have mediocre duels until you can hit your power spikes. However, if you outpath your opponent and get those early leads, you should snowball the game hard. As of patch 8.14 he has a decent 49.3 win rate.

Runes are fairly simple. I'll start with the popular pick first: Predator. The Predator keystone can be switched out for the Electrocute keystone for a better winrate of 50%.

Predator to move fast. It has a high pick rate, but a sub-50% win rate overall.
Sudden Impact for that lethality;
Eyeball Collection for that extra AD;
Relentless Hunter for that thicc movespeed;
The secondary runes are Celerity and Waterwalking from the sorcery rune page.

Next I'll go to the Phase Rush set of runes. This set has a 52.6% winrate overall.

Phase Rush for that movespeed;
Nimbus Cloak for that movespeed;
The standard Waterwalking- Celerity combo for that movespeed;
And with Sudden Impact and Relentless Hunter from the domination page.

Following these, we have the Aftershock and Grasp of the Undying trees.

Aftershock for that tankiness;
Bone Plating for those ganks, so you take no damage;
Chrysalis for that little bit of extra health;
And finally Revitalize to jive with your w, Spirit of Dread.
The secondaries are Sudden Impact and Relentless Hunter from the domination page.

Of course, these runes are not set in stone, and these can be swapped for different runes or secondaries. For example, you can switch your secondary trees to the Inspiration tree to get Magical Footwear and a Time Warp Tonic to get some extra AD.

This is where people may be confused. Most Hecarim players won't take Flash. It's because with your r, Onslaught of Shadows, being unstoppable, you don't need a flash. Instead you can take something more useful like Ignite or Ghost. Of course, this is in combination with Smite if you're in the jungle.

Some pros of Hecarim include:

-He's fun to play
-He goes real fast
-He has unique abilties
-He has a unique playstyle
-Once you scale, he does a lot of damage

Some cons of Hecarim include:

-He can get shut down easily
-Slows hurt so much
-Takes so long to scale up to the mid- and lategame
-Does no damage without items
-Can have a hard time in certain matchups

There's not much to say in terms of items here, seeing as Hecarim doesn't actually have great scaling on any of his abilities, meaning his abilities are more for utility. Therefore you want to build him off-tank with a few damage items that synergise with his kit overall.

Because Hecarim has low health and other base stats, he usually wants to rush a Trinity Force. The only problem with this is that a TriForce is one of the most expensive Items in the game, forcing him to be extra conscientious about his farming and how much time he wastes anywhere.
Hecarim's next item is the standard Enchantment: Cinderhulk on a Skirmisher's Sabre. This works well with the rest of his items that he builds. The reason he doesn't build Enchantment: Warrior is because his scaling on his abilities is mediocre at best.
Sometime in between building your cinderhulk and your triforce, you want to get upgraded boots. This will change depending on how far ahead you are in the matchup. Most often, you'll be buying Ninja Tabi.

After you have your core 3 items, you have some options. If you're super ahead, you can go ahead and build damage items like a Youmuu's Ghostblade or a Rapid Firecannon. Other options include a Stormrazor or a Duskblade of Draktharr. If you're even or behind, you'll want to build resistances. These include a Dead Man's Plate, a Spirit Visage, a Turbo Chemtank, a Maw of Malmortius, or situationally, a Randuin's Omen. You may also want to stick a Guardian Angel in that build somewhere if you feel that you'll need it.

With Hecarim you'll be maxing your q, Rampage, because it has the best scaling, and you'll be using it the most. It also has the shortest cooldown, and no cast time, so if you're running into someone with your e, Devastating Charge, it won't be interrupted.

Following your q max, you max your e, Devastating Charge, because that's your engage tool. It also does a lot of damage.

Last and certainly least, you max your w, Spirit of Dread last. If you're fighting champions with it active, you heal for a flat percentage, that doesn't increase with level. It also has a garbage AP scaling, so there's no point in maxing it at all.

Maxing your r whenever you can, Onslaught of Shadows, is standard.

Once you have some items, you can take duels wherever you want. However, with nothing, you want to avoid your enemy if you don't have your team to back you up.

In the jungle, you want to remember, you need lots of gold for that Trinity Force. So you gotta get your gold quickly, so you can't waste any time on inefficient pathing. I have a guide on how to jungle well here. Once you do your first clear, your ganks should be pretty easy, you have substantial displacement with your e, Devastating Charge. You knock your enemy into your team and get a kill.

When you do your first crab, you want to try and get the crab furthest away from the enemy unless your team is there to back you up, because you will lose virtually every level 2 duel. Most of the time, you can grab the top crab uncontested. Try to do that.

As Hecarim, you're probably going to be a tank, so you want to peel for your carries behind you while dealing damage with your Trinity Force-aa combo. If you're the carry, you're going to want to try and flank into their backline and take out their carries with your Devastating Charge. At some point during most teamfights, you want to ult through their entire team, and cc-ing as many enemies as you can to deal the most damage to their team.

This is a beautiful example of a perfect Hecarim ult.

Credit for ult is here.

Other times you'll want to flank the enemy and look for their carries even if you are the tank, especially if you have another teammmate peeling for your backline. That means you have the opportunity to displace or kill the enemy damage dealers.

I am by no means the best Hecarim player, and I probably never will be. This however is a comprehensive guide to a champion that's fairly arcane. I have other guides as well, my Kayn guide can be found here and my jungling guide can be found here. Best of luck in your next games!