hecarim is a strong melee champion,good ganker, good game changer, good jungler, and other stuff. i don't really know how to start this so blah blah blah. you get the picture.

why armor pen marks? so u can easily kill soft champ.

Why armor seal? hecarim is too soft at early game so you need defense from ad champ.

why magic resist glyph? for more defense against ap champ.

why armor pen quintessence? so u can op the tanky champ in early game.

I took 9-21-0
i took the armor pen in offense and of course for ads and ad
and for defense i boost up armor, magic rest, and other stuff for more tanky.
i focus on defense because hecarim is to soft at early and easy to gank if you have exhasut.

I start with boots and 3 health pots after that i still have 20 gold so i w8 for the 4th pots or you can go and dont w8 for it. then if i have enough gold i recall and get heart of gold or philosopher stone ( you still need that 2 items because they grant you gold and give health regen, mana regen, and hp). after that I get the phage for more hp and ad and slow enemys. then get the berserker greaves for ads ( you can get merc threads or ninja tabi if need defense). then i get frozen mallet. then just continue the items on the top. then after getting pd you can change the zeke herald to blood thrister.

(getting tried of typing)
I pick ignite and flash for easy first blood.
you dont really need ghost because hecarim has his 3rd skill for move speed.
you can pick exhaust to if you want. its good against ad champ.
you can even use teleport if you want to jump into battles more easily and easily roam the map.

(Finally last Part)
This hecarim guide is kinda all around build. you can be tanky and can be a good hitter champ.
Thanks for reading hope you like it sorry if it sucks or anything just leave a comment and i'll answer your question. thanks and bye.