Pokémon X and Y Walkthrough, Pokémon Move Sets: Ferrothorn
(Please note that the recommendations below are largely made for in-game play. A Ferrothorn in a competitive environment will likely make use of different moves to accommodate for smarter, more adaptive opponents. If you have your own Ferrothorn build to share, let the world see it in the comments.)
Grass-types aren't often known for receiving a great deal of hype in the competitive community, but Ferrothorn is a massive exception. It has an incredible mixture of solid typing, good stats, great moves for its role, and good abilities. This pokémon looks means, and it can be every bit as vicious in battle as it appears.
Type: Grass / Steel
Simple one, here. Ferrothorn starts off as Ferroseed. Level it to 40 and there will be a Ferrothorn in your party.
Dragon, Electric, Fairy, Grass, Normal, Psychic, Rock, Steel, Water. Immune to Poison.
Fighting, Fire (4x).
Ferrothorn is every bit the defensive beast it appears to be. Its Defense clocks in at a hearty 131, which is generally more than sufficient for warding off even fighting-type moves. Special Defense is second with a nice 116, Attack is third at 94, and HP comes fourth with 74. Special Attack is a pitiful 54, which is no big deal, and Speed... well, this thing's Speed is just about as low as you'll see, with a base 20. Fortunately, you can make Ferrothorn's rotten Speed work to your advantage.
Iron Barbs: Physical attacks used on this pokémon with do a small percent of return damage to the attacker. This is Ferrothorn's only possible ability, and quite frankly you shouldn't want rid of it anyway - so many pokémon will be forced to chip ineffectually away at its hide that you'll be doing virtually the same damage in return. This amount is amplified further if you give Ferrothorn a Rocky Helmet, which isn't a bad idea at all.
Ferrothorn can few different directions, one of which is a potent wall. It has two moves which will provide ample STAB damage against a variety of opponents: Gyro Ball, which takes advantage of Ferrothorn's slow Speed, and the inaccurate-but-horribly-powerful Power Whip. You may miss occasionally with the latter, but it's less of a problem with Ferrothorn in the lead. Spikes or Stealth Rock provide a painful distraction that you can set up at the beginning of each round (I prefer Stealth Rock for bringing Charizard Y to its knees), and Thunder Wave is just plain annoying. If you want increased combat prowess, substitute Curse over Thunder Wave instead.
Ferrothorn also has the capacity to stall effectively. Leave it one STAB move for emergencies, then give it Toxic to slowly chip away at health. Beyond here you can either deploy moves that will slowly eat away at your opponent's HP (Leech Seed is fun, and Spikes / Stealth Rock remain a possibility) or you can use moves that will bolster or shelter Ferrothorn's HP, such as Ingrain and Protect.
EV / Super Training
As per usual, this depends on the route you're taking with your Ferrothorn. An attacker will want to pour the lion's share of their EVs into Attack and HP. A defender should eschew combat prowess and instead emphasize survivability with a full share of EVs going into HP and the rest split between Defense and Special Defense. The fire-type moves that are so painful to Ferrothorn seem to be leaning more to the physical in this generation, though you will still see the occasional Flamethrower, so use your own discretion on which side gets more EV attention.
Catching a Ferrothorn
Ferrothorn do not appear in the wild in Pokémon X and Y, but their predecessor Ferroseed does. You can catch one in Reflection Cave by targeting the darkened spots on the ground.