For a long time, there was no true ‘ultimate tank’.  Skarmory put out a lot of effort and a good showing, but electricity could still snag it from the sky and anything packing a fire move just ruined its day.  Clefable could pull some terrible Cosmicpower/Wish tricks with its Magic Guard.  Shedinja just makes things cry- if they haven’t got weather effects, and a properly trained Forretress or Bastiodon could take a lot of punishment- but both had serious weak points that were pretty easy to reach.

Then fifth gen hit, and Ferrothorn was unleashed.

During the first spoilers, the strange plant got little notice- it was another especially weird-looking thing in an array of a lot of new weird-looking pokemon.  But then people started to realize what this bizarre plant was really up to.

Ferrothorn was about to turn the metagame on its head.

When we speak about the metagame, we’re talking about the game outside the game- the environment of other people you play with and how you as a player react to them and their choices.  For a long time walls were annoying, but without special care, they really weren’t something to be outright feared.  Even then, there was always an easy answer- Cross Chop that Clefable, retain some lighting for Skarmory, and who actually used Forretress or Bastiodon?  They couldn’t give out a serious threat in any notable way (though Forretress could be fun on an entry-hazard team and useful for its Rapid Spin).

Ferrothorn, however, overtook all the other walls- by virtue of a few small factors that added up into one big problem.

Steel/Grass is a hideously durable typing- while it has a major weakness for Fire and a lesser weakness to Fighting attacks, there’s nothing else that it has weakness to.  On its own, this would put Ferrothorn on a level with Forretress- interesting and useful, but hardly remarkable.  However, being a Grass-type, Ferrothorn gains access to Leech Seed and Ingrain, securing it in place against Whirlwind or Roar and combining together to give it a fearsome level of constant recovery.  To make things worse, where classical walls like Ferrothorn have a weakness in one defense stat (Blissey’s Defense, Skarmory’s Special Defense), Ferrothorn itself packs an over 100 rating in both defenses, and hasn’t got the low HP that usually go with that (see Bastiodon).  What’s worse, while its speed is terrible (which mostly just enables it to whip out horrifyingly powerful Gyro Balls), its Attack is actually a very respectable 94 rating (which only serves to make Gyro Ball that much more powerful).

That’s just the start.  On top of this, Ferrothorn can wield not only Spikes but also Stealth Rock, which is usually found to be the most dangerous entry hazard in the game.  Power Whip gives it a horrifyingly strong physical attack of the Grass type, Explosion and Selfdestruct, while usually wasteful on something with so much recovery, are an equally terrifying prospect from the powerful plant, and tacking Swords Dance onto the thing runs the situation into even more ridiculous territory.

AS if all of that weren’t enough, Ferrothorn also has the Iron Barbs ability- anything that touches it in combat takes damage.  This lets out most of the Fighting moves that would hit its secondary weakness, since most anything coming in to use one will likely be taking a hit not only from Stealth Rock, but also from Ferrothorn itself- which can be fatal pretty easily.

Previously, Fire was a convenience- most things with Fire weakness had other weaknesses to exploit, and the only wall that really needed it at all was Skarmory- and with Stone Edge and Electric attacks everywhere, Skarmory just wasn’t threatening enough on its own.

With Ferrothorn on the field though, Fire attacks are virtually a necessity in a fifth generation team- which not only breaks the lock that Boltbeam had on most teams’ movesets, but also changes the whole field, as having something that not only wields Fire attacks but resists Grass and Steel moves (i.e. is Fire or is Steel itself) has become much more important, giving the often-maligned Fire type a good place.

Just the possibility of running into Ferrothorn has changed how the game is played- which is why you can’t really ignore -anything- going around.  Sometimes the strangest things have the biggest effects.