Hecarim is a new addition into the league. He is a ghastly centaur and a melee fighter with a movement speed focus. He can be played many ways, as a tank or as a bruiser, and in lane and in jungle. I believe Hecarim is best played as a initiator tank from the jungle. He combines aspects of feared junglers such as Skarner, Shyvana and Rammus, which makes for a great jungler and even greater ganker.
Runes on Hecarim are mostly simple. I go with a basic tanky dps setup with attack marks, armor seals and magic resist per level glyphs. I finish with attack damage per level quints, but movement speed quints would be a good choice as well. I like this setup because of its focus on early game jungle power. The attack damage and armor will help clear camps more quickly and safely, while the magic resist comes in handy later in the game.
For masteries, I go 0/21/9, taking speed, defense and extra buff duration. This setup is very well suited for Hecarim. It helps with tankyness, something that is needed for him to do well early on, and also assists his mobility.