Hecarim is not viable.
I just don't believe RIOT put any real effort to him as champion, he has been built purely for in the jungle with poor dueling (giving him no presence in lane) and has been given a sub-par kit with poor ratios . The worst part about Hecarim though is he isn't even strong in the jungle, they place caps on all his abilties related to minions ( with massive CD's) making his clear times slow and his sustain just plain awful. Lets not forgot his ganks, nothing special....outclassed by every other champion...
Let's sit down with riot for a minute... "We are going to make a champion that is outclaseed by every other champion league of legends and is not viable in any position. To do this we made it so he had a horrific early game and a poor duelist, this is so he has no lane presence and can ONLY just barely be played in the jungle."
Hey he may have an ok late game, but it's nothing to ride home about. He still get's outclassed by most champions in lol..
Why do you think he isn't used in competitive play? He has way too many short-comings and can not 1v1 a fly...
...im assuming the pros wouldnt use him if he wasnt viable