The Pony

As a Hecarim main pre his popularity:

He's 100% melee. Taking a ranged harass champion against him does wonders. Take a slow-oriented team - he has a large hitbox and slows will reduce his bonus AD (skillshot champions do well against him in general). Keep track of his cooldowns, only his Q has a low cooldown, his E and R make the big plays and he can be targeted when they're down. Keep track of when teleport is available as the homegaurd teleports are hugely impactful plays. His R is HUGE in teamfights; making him use it defensively to escape will reduce his pressure immensely. Keep lots of more distant vision - he ganks by closing distance quickly so you need to react to him earlier than most. You can flash as his E connects to throw out extra distance.