Ferrothorn Description

Ferrothorn, The Thorn Pod Pokémon. 

It fights by swinging around its three spiky feelers. A hit from these steel spikes can reduce a boulder to rubble.They attach themselves to cave ceilings, firing steel spikes at targets passing beneath them. By swinging around its three spiky feelers and shooting spikes, it can obliterate an opponent.
Overview
Ferrothorn was easily the best non-legendary Pokemon to come from Black & White. Havin stellar defenses, a good typing, an amazing movepool with moves like Stealth Rock, Spikes, Leech Seed, and so forth, making it one of the best support Pokemon in the entire game. Although weak to common Fighting and Fire moves, most of the time Ferrothorn is able to shrug them off, lay hazards, hurt the opponent with Iron Barbs, or cripple them. The only problems with Ferrothorn are its dismally low Speed, and no reliable recovery, but it makes up for these shortcomings by being the best defensive pivot in OU (if not the game).

Abilities
Iron Barbs: When a physical move comes into contact with Ferrothorn, the attacker takes 18% damage. This is perfect for deterring a physical attacker from trying to whittle Ferrothorn down, as it'll be chipping away at its health as well
Movesets
The Thorn In Their Paw
-Spikes / Stealth Rock
-Power Whip / Protect
-Thunder Wave / Gyro Ball
-Leech Seed / Stealth Rock
Item Attached: Leftovers
Ability: Iron Barbs
EVs and Nature:
EVs: 252 HP / 48 Def / 208 SDef
Relaxed Nature (+Def, -Spe)
This moveset is basically what you'll see on every Ferrothorn ever. The first slot goes to some hazard or other, Spikes generally since not too many Pokemon get them, but you can run Stealth Rock if you don't have another Pokemon that sets it on your team. The second slot mainly depends on what else you run. Power Whip gives Ferrothorn some reliable STAB as well as giving it a way to really hurt the Water types it completely walls. Protect is generally used for scouting out what coverage the opponent has or what they'll switch in to counter Ferrothorn, but it can also be used to stall with Leech Seed to rack up a lot of health quickly. The third slot either goes to Thunder Wave, which is great for crippling opposing sweepers, or Gyro Ball which serves as extra STAB boosted by the fact that Ferrothorn is pretty darn slow. The last slot either goes to Leech Seed (pretty much your only source of recovery) or Stealth Rock if you want to run double hazards. This is easily the most common set for Ferrothorn (or some variation of this), so be prepared for it as it is quite common.
Pumping Iron
-Power Whip
-Gyro Ball
-Bulldoze
-Explosion / Spikes
Item Attached: Choice Band
Item Attached: Life Orb
Ability: Iron Barbs
EVs and Nature:
EVs: 252 HP / 252 Atk / 4 SDef
Brave Nature (+Atk, -Spe)
This set is a little unorthodox and definitely unexpected. Power Whip acts as primary STAB, hitting most everything weak to it for high damage. Gyro Ball is another STAB option that hits faster Pokemon for extreme damage (don't overuse this though as it only has 8 PP maximum). Bulldoze takes care of opposing Steel types which resist both of Ferrothorn's STAB moves, as well as handily lowering their Speed, leaving them easy bait for anything switching in. The last slot is an interesting choice, with Explosion doing quite a bit of damage to something you can't take out otherwise, while Spikes allows you to pretend to have the normal set (as well as lay hazards), hopefully adding more to the surprise factor.
Other Options
Gravity, Ingrain, Iron Head, Payback, Seed Bomb, Substitute, Swords Dance, Toxic
Gravity is a useful support move that Ferrothorn can run, although very situational as Ferrothorn can't really take advantage of it itself.
Ingrain provides recovery, although the moveslot to use on it would be difficult to choose as it really doesn't heal that much and it only applies to Ferrothorn (unlike Leech Seed).
Iron Head is an alternate Steel STAB that has a reliable damage output as well as more PP, though you won't be flinching much outside of Shuckle with this.
Payback takes advantage of Ferrothorn's abysmal Speed stat and ensures that everything will hit Ferrothorn first, meaning Payback does more damage. Unfortunately, the coverage Dark provides doesn't help Ferrothorn too much.
Seed Bomb is an alternate Grass STAB option, with better accuracy than Power Whip but a lot less power.
Substitute is rather situational on Ferrothorn as most Pokemon will be able to do what they want before it sets up a Sub, but if it can get it on a switch, it can generally hide behind the Substitute for a while as it is quite bulky.
Swords Dance is here for the kicks and giggles, and mainly to make your opponent question what exactly you think you're doing. Have fun with it though, it could work if you're lucky.
Toxic can be quite useful on Ferrothorn as it helps to wear down other walls which Ferrothorn couldn't put a dent in otherwise, although once again worrying about what to replace on its moveslot.
Double & Triple Battle Options
Ferrothorn essentially fills the same role it does in Doubles and Triples as it does in Singles, just sitting around being an extremely fat wall that's hard to take down. It also handles the bulky Waters that are more prevalant in the format such as Jellicent, Swampert, and Gastrodon.
Partners
Politoed is a great partner for Ferrothorn because of its ability Drizzle, as it summons (almost) eternal Rain which reduces the damage that Ferrothorn will be taking from any of those 4x super effective Fire attacks. To be perfectly honest though, Ferrothorn is mainly what you'll be looking at to slap on a team to fill some hole or other, not to just build around it.
Countering Ferrothorn
If it's not in Rain (or even if it is), most anything Fire type can take a pretty big chunk out of Ferrothorn's spiky health bar. Ninetales is probably one of the better ones as it sets up Sun (meaning more damage) and like all Fire types resists both of Ferrothorn's STABs. Heatran and Infernape are also good options for firing off Fire attacks, though there are many Pokemon that run Fire coverage moves (Latios, Jirachi, Hydreigon, etc) that can also do major damage. Strong Fighting types also give Ferrothorn some trouble, such as Terrakion, Keldeo, Breloom, and Conkeldurr. Tricking Ferrothorn any Choice item (as long as it isn't the Band set) will generally mean bad things for Ferrothorn. Taunt users also do well, preventing it from laying hazards, Leech Seeding, or trying to Thunder Wave.

Pre-Evolution Corner - Ferroseed
It Came in like a Wrecking Ball
-Stealth Rock / Spikes
-Leech Seed
-Thunder Wave / Protect
-Bullet Seed / Seed Bomb / Gyro Ball / Spikes
Item Attached: Eviolite
Ability: Iron Barbs
EVs and Nature:
EVs (RU): 252 HP / 252 Def / 4 SDef
EVs (LC): 164 HP / 188 Def / 148 SDef
Relaxed Nature (+Def, -Spe) or Sassy Nature (+SDef, -Spe)
Ferroseed inherits all the traits from its firmly-planted Overused father, Ferrothorn. It provides much support to a team: with its access to both Stealth Rock and Spikes, stalling potential with Leech Seed, an amazing ability in the form of Iron Barbs, killer defenses, decent attack, and good typing (which is getting a huge hit next gen due to Pokemon X & Y’s revamped type chart), Ferroseed becomes the wall of walls. This little ball of spikes is a persistent threat in the entire game. Stealth Rock and Spikes are the crux of Ferroseed and Ferrothorn. Stealth Rock hits all Pokemon for damage based their Rock-Type weakness / resistance upon entry, while Spikes hits all Pokemon that touch the ground and can be put up for multiple layers. Stealth Rock is usually more useful, as it hits all Pokemon, and Ferroseed will not usually have consistent time to set up the layers of Spikes in Little Cup due to its fast-paced matches, though Spikes are still perfectly viable in RU. Leech Seed lets Ferroseed stay healthy, while damaging the opponent. Thunder Wave shuts down many Pokemon, as lots of the more viable Pokemon in Little Cup usually have decent speed to boast. Protect lets you scout and stall for Leech Seed damage and recovery. Bullet Seed lets you attack for decent damage that can burn through Substitutes, and especially good for dealing with Chinchou in LC. Gyro Ball makes use of Ferroseed’s abysmal speed and the tactic of fast Pokemon in both RU and Little Cup. If using Gyro Ball, remember to convert Ferroseed’s Speed IVs to 0. Eviolite further bulks up Ferroseed’s incredible defenses; the EVs do the same thing, while helping it’s HP. Most fire attacks will obliterate Ferroseed outright. Rapid Spin rids your field of Stealth Rocks, Leech Seed and Spikes. Ferroseed will probably make you rage a lot, if not dealt with properly, but I don’t think it really cares. Ferroseed serves pretty much the same purpose in both RU and LC, acting as a mini Ferrothorn and blocking most Pokemon just as efficiently.

Locations in Games
Ruby/Sapphire/Emerald:
Not in game
FireRed/LeafGreen:
Not in game
Colosseum/XD:
Not in game
Diamond/Pearl/Platinum:
Not in game
HeartGold/SoulSilver:
Not in game
Black/White:
Evolve Ferroseed
Black 2/White 2:
Evolve Ferroseed