This is a guide I tried out just for ***** and giggles and it turned out to be fun and very viable. In general Hecarim is a champion who is best played as an unseen threat. Ward coverage and removal is an important role for any Hecarim build and is in general a game winning factor.
Furthermore this build is experimental and is still not purposed for people who like dishing out tons of damage and still being able to cover tanking roles. It is a very expensive build and requires that you buy Philosopher's Stone as fast as possible.

For reference:
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Pros [+] High damage [+] Insane ultimate [+] Extreme sustain [+] Can close big distances with Onslaught of Shadows [+] Extremely fun to play [+] Viable tank

SPACE

Cons [-] Vulnerable to cc, especially slows which reduces his damage [-] Early levels are crucial for Fiora [-] Tends to fall behind fast if you don't gank or kill. As any other jungler

The choice of runes and masteries is focused around the Hextech Gunblade which combined with Spirit Visage gives Hecarim a great sustain in combat. Since Spirit Of Dread will be your main skill for damage and sustain.

In this build you will be focusing on having Spirit Of Dread as your main damaging and tanking ability.
The best use of Hecarims ultimate is to save it to counter enemy initiates focusing your carries, escaping teamfights or catching people out of position. Many do the error by using him to initiate teamfights with the ultimate which leaves him extremely vulnerable and open to cc

Still not done, but it's a work in progress :P