There are pokemon who initially seem like they are one of the Big Four varieties, but turn out to have the move support needed only for one way of functioning as that variety of pokemon. This can be because they need the help of their moveset so badly to be competitive that they haven’t room for options, or because they lack the needed moves to enable full function. They will often have the stats needed to qualify as a good Tank, Wall, Sweeper, or Trickster- but will only be able to perform the function with a small selection of moves, the rest of their options poor moves or not fitting that position.
An excellent example of a specialized wall is Dusknoir. On first glance, Dusknoir looks like it might be either a Tank or a Wall, with its massive defenses and strong Attack stat. A quick look at its move options lets it out of the Tank category- it lacks the generally strong moves and overall survivability that will let it chug along for greater periods of time, and its primary Ability (Pressure) is far more suited to a Wall than to a Tank. However, it has a few crippling weaknesses as a Wall that restrict it. Poor HP mean that Dusknoir can’t sustain its defensive in the face of Night Shade, despite its immunity to Seismic Toss (and indeed, all Fighting and Normal attacks). It doesn’t have the moves needed to boost its defenses, and while it does have access to Will o’ Wisp, Toxic, and Curse, it’s still a bit shy on ways to really chip through much, having poor coverage options in its attack types. However, it does have Pain Split and low HP, making it an excellent wall to pit against high-HP walls like Blissey or Drifblim. Its excellent defenses let it keep a certain amount of durability in nearly any situation, but it has no way to restore its own HP outside of Pain Split. While it can perform as a wall with Pain Split and possibly Drain Punch or a Shoal Bell (though those options aren’t terribly good), without Pain Split Dusknoir tends to be rather lacking and gets relegated to the unremarkable position of a semi-competent and rather slow tank. Dusknoir has access to things like Gravity, Curse, Will o’ Wisp, and Thunder Wave, but not enough to actually perform as a Trickster (nor the neccessary speed to do so either)- much like most pokemon that are straight-up Tanks, Sweepers, or Walls.
Most pokemon that don’t fit well into the Big Four varieties are relegated to lower tiers, because difficulty of use and a need for either prediction or protection make them much harder to use on a team- a lucky swing on your opponent’s side, or facing a team entirely of things that are good as answers to that pokemon will leave it useless except for taking one or, if you’re lucky, two power points off of some move your opponent is fond of.
While it can be a bit of an effort to back such a pokemon up enough with the other team members, it can be worthwhile when a pokemon your opponent is not expecting or unused to encountering suddenly tears a hole in their team, whether it’s because of your planning to take into account their weaknesses, or just the sheer surprise factor.
