Underused Game Mechanic / Pokémon

Pokémon has Underused Game Mechanics in spades, with the creators admitting that many mechanics and features are made with the idea that such things are going to be unique to that specific region or generation, in order to help differentiate them from other installments. Hence things like only Hoenn having secret bases, or replacing Gen VI's Player Search System with Gen VII's Festival Plaza.

Gen I:

• The Cut HM can be used to remove patches of the tall grass wild Pokémon appear in, allowing the player to avoid getting Random Encounters. However, the process of using the move repeatedly is time-consuming enough that just proceeding normally will get you where you're going in either the same amount of time or even faster if you get minimal encounters. Additionally, the mechanic is rendered redundant by the existence of Repels, which have the additional bonus of not requiring a Pokémon with a specific move in your party to use. The mechanic was expanded on in Emerald via the Hyper Cutter ability doubling the radius of the removed grass patch, but this property of the ability is never explained in-game. Following Gen III, the ability to cut grass patches was removed entirely.
• The Generation I games played around with the idea of the midgame being open-ended, allowing the player to explore much of the region and get several badges in whatever order they wish. For a long time, the Gen II games and the remakes for both these generations were the only time this type of plot progression was present, until Gen IX embraced being a Wide-Open Sandbox from the get-go.

Gen II:

• Gold and Silver's postgame allows the player to journey to Kanto and see how it's changed since Red's adventure, which is still considered by many fans to be one of the best approaches done for a Pokémon endgame. However, the idea of traveling to a past region has yet to be revisited outside of these games' remakes. (Gen IX technically has the player travel to Unova as part of its DLC... but you're specifically going to a school located far offshore of Unova itself, so you never actually go to any locations seen in the previous Unova games.)
• Having the player character's mother save some of your money after every trainer battle was a neat idea and gave a sense of security if you happen to lose a battle (which resulted in you losing money). Other than saving up for a rainy day and having your mother buy random items and dolls for you, you could play the entire game without sending money to your mother without it affecting your gameplay. Save for the remakes, this mechanic hasn't been reused since.

Gen III:

• The Vs. Seeker, introduced in FireRed and LeafGreen, allows the player to re-battle any trainer they've already fought and face higher-level teams. This makes leveling up the player's Pokémon much easier and permits grinding for more money. However, the Vs. Seeker is absent in all games after the Gen IV Sinnoh games.
• The Battle Frontier, introduced in Emerald, has only seen two incarnations across three games (Platinum and HeartGold and SoulSilver used the same Battle Frontier). The Gen V games onward use the Battle Tower template for their post-game battle areas, lacking the unique gimmicks of the other attractions; infamously, the remakes of the Gen III games omitted the Battle Frontier in favor of recycling the Battle Maison from Pokémon X and Y. Pokémon Ultra Sun and Ultra Moon features a Battle Factory-esque area, but doesn't bring back any other attractions.
• The Shadow Pokémon from the spinoff Orre duology have only appeared in those games, and they cannot be transferred to the mainline Gen III games until they are purified, removing their Shadow traits. Most of this is for balance reasons, as Shadow Pokémon's mechanics make them very overpowered (Colosseum gave every Shadow a move that ignored the target's type and could randomly put them into a Critical Hit Class mode; XD includes many different Shadow moves, which can either be physical or special and are super-effective against anything that isn't also a Shadow). Shadow Pokémon do appear in Pokémon GO, but without Shadow moves.

Gen IV:

• Starting in the fourth-generation games, many Pokémon species were given minor sprite differences for each gender, such as female Pikachu having heart-shaped tails while males kept the flat-gopped tails. This was the only time gender differences were applied on a mass scale, as future games typically only have one or two Pokémon with notable dimorphism between genders (such as Unfezant, Meowstic, and Indeedee). Later games still maintain gender differences, and Pokémon Let's Go, Pikachu! and Let's Go, Eevee! even retconned Eevee to be one of the Pokémon with one, but it's almost an artifact since very few species introduced past the first four generations have differences. In fact, gender differences aren't even as expansive as they were initially planned; leaked prerelease builds of Diamond and Pearl show that way more Pokémon were planned to have them, such as female Charizard only having one horn.
• HeartGold and SoulSilver, in order to make up for the lack of the Pokétch, fills the bottom screen with shortcuts to menu options and an interact button. There's one button that toggles your Running Shoes so that you can run without holding down the B button. Since there's few times when running is a bad thing, this is very useful. However, as later games lessened the use of the touch screen, this toggling feature never returned. However, Pokémon titles on the Switch don't need it anyway due to running being tied to how far the analog stick is pushed in a given direction.
• Key items exclusive to HeartGold and SoulSilver include Berry Pots (which let you grow berries on the go, instead of planting them in a single spot and having to remember where they are), and the GB Sounds device (letting you replace the remake's soundtrack with the soundtrack from the original games, a feature absent in Omega Ruby and Alpha Sapphire but returned in Brilliant Diamond and Shining Pearl with the DS Sounds).
• Amity Square lets players walk around with a cute Pokémon following them...by which they mean "Pokémon that had overworld sprites drawn for this game". Platinum expanded the list to include the Sinnoh starters and their evolutions regardless of whether or not they're meant to be cute, and then HeartGold and SoulSilver not only had overworld sprites for every single Pokémon, but also had the first one in your party follow you everywhere. This feature would not be seen again until Pokémon Let's Go, Pikachu! and Let's Go, Eevee! to the disappointment of many, and then leave again until Pokémon Sword and Shield had a similar feature exclusive to the DLC areas.

Gen V:

• Pokémon Black and White introduced two new types of battles: Triple Battles and Rotation Battles. Triple Battles, as the name suggests, have three Pokémon active on each side at once; unlike Double Battles, positioning matters because Pokémon on one end generally can't do anything to the Pokémon on the other end, although the ability to switch the position of the middle Pokemon with the ones on the ends adds an extra layer of strategy. Rotation Battles are similar in that each side has three Pokémon out, but only one is active at any given time; in addition to attacking or using items, each side cab rotate their active Pokémon out as a free action, making Rotation Battles a major mind game. Neither battle variant has very many NPCs that use them, and they never caught on competitively outside Japan. Another factor is that having half a dozen Pokémon on the field produced issues with lag, especially after the franchise made the Video Game 3D Leap in Gen VI. This resulted in both battle types being dropped by Gen VII.

Gen VI:

• Pokémon X and Y introduced Inverse Battles which, as the name suggest, that inverts the series' Elemental Rock-Paper-Scissors (Fire is strong against Water instead of resisted, Ghost can now hit Normal, etc.). While this was an interesting new take that drastically changed how almost every Pokémon functioned, breathing new life into otherwise horrible ones in the process, it could only be used against one optional NPC. And there was almost no Player Versus Player support for it. Omega Ruby and Alpha Sapphire did expand their availability somewhat by letting players set their Secret Bases to use Inverse Battles, but that was about it. Just like triple and rotation battles, the following generation dropped them completely, though Sun and Moon oddly has an Inverse Battle quiz available in the Festival Plaza despite them no longer existing.
• X and Y also introduced another kind of gimmick battle, Sky Battles. Like the name suggests, Sky Battles take place high in the air, limiting the participants to using only Pokémon that are Flying-types or have the Levitate ability, and using moves that don't involve some any kind of terrain. While Sky Battles were more common than Inverse Battles (or Triple and Rotation Battles, for that matter), they were still all optional and not eligible for Player Versus Player matches. Furthermore, it wasn't very intuitive on which Pokémon would actually be eligible to participate; if the Pokémon's idle animation had it standing or sitting on the ground, it was disqualified regardless of its typing or ability. Apparently having two sets of idle animations for a given species wasn't an option, because a number of species got unpopular changes to their previous idle poses to make them eligible for Sky Battles; unfortunately, while Sky Battles never made it out of their game of origin, the idle pose changes have persisted for much longer.
• The move Flying Press deals damage as if it was Fighting and Flying type simultaneously. The idea of dual-typed moves had a lot of potential to shake-up matchups, but not only is Flying Press a Secret Art exclusive to Hawlucha (which typically has better Fighting and Flying options), no further dual-typed moves would be introduced since. The move even had its power raised in the very next generation because it was that overlooked.
• Omega Ruby and Alpha Sapphire have Horde Trainer Battles. These were hyped up a fair amount in the trailers, and it felt very in-character for villainous team grunts to gang up on the player. Unfortunately, there are a grand total of two Horde Trainer Battles in the entire game: one in the Team Magma/Aqua Hideout during the main story, and one in the postgame Delta Episode. There aren't any repeatable battles that use this mechanic, and it isn't available for PvP.
• Mega Evolution was a big selling point for Gen VI as a whole, and about half of the game's plot focuses on it. Despite this, there's a decent chance you won't end up using a Mega Evolving Pokémon on your final team. In X and Y, only two Mega Stones are given to you outright (your Kanto starter and Lucario), and there are only four other stones^note plus one with the event-only Blazikenite, plus another two with the other Kanto starter stones you can buy that you can get during the main game. The other seventeen or so can be gotten in the post-game, which is infamously short by Pokémon standards. Omega Ruby and Alpha Sapphire fixes this; in addition to adding more Mega Stones, you're able to get over twenty of them during the course of the main game (though you're only able to use Mega Evolution for about half of it).
• Related to the above, only three storyline opponents use Mega Evolution against you, one of them being a simple tutorial for the mechanic, one being the Big Bad, and one being the Final Boss. This is in spite of its hype in the story. Later generations seemed to acknowledge this, with their respective gimmicks being used by every boss-level opponent.
• You can ride Pokémon in X and Y, for the first time outside of Surf. However, there's only three to ride on, all are limited to one area each, and two of them are slow and cumbersome to control, so it feels like a throwaway gimmick. The Gen VII games would properly implement this with the Pokémon Ride system.

Gen VII:

• Pokémon Sun and Moon caused this to happen to Mega Evolution again. While it makes sense since Sun and Moon have a host of other new mechanics to take the center stage (i.e. Z-moves, Ride Pokémon, etc.), fans were still disappointed that no new ones were even introduced. It doesn't even get any significant mention until the post-game, and you only get a single one (Alakazamite) for free. To make matters worse, more than half of them weren't even available in the game at all until Game Freak released them through special events. The rest must be bought with Battle Points you earn playing in Battle Royales or the Battle Tree. For how much it was promoted both in the games and the anime during Gen VI, the Mega Evolution concept's shift from a hyped new mechanic to being an afterthought in the very next installment of the series is somewhat jarring. The Updated Re-release at least makes all Mega Evolutions available (though some are version exclusive) but there are still no new ones. Pokémon Sword and Shield would end up removing Mega Evolution (and also Z-Moves) entirely in favor of Dynamax.
• Sun and Moon brought back Johto's Apricorn Balls much to the surprise of a lot of fans... until they realize that it's only possible to obtain one of each ball. It's a wonder why the makers couldn't add some sort of stall or shop in order to purchase more.
• Some have argued that Sun and Moon's picture-taking mini-game doesn't quite reach the complexity and uniqueness that defined Pokémon Snap. Slightly fixed in Pokémon Ultra Sun and Ultra Moon, which increases its usability.
• After their popularity during Gen VI, it was a bit of a surprise that Sun and Moon dropped Horde Battles completely; the mechanic would have fit in perfectly, since Pokémon from Alola are said to help each other out a lot. Instead, it got replaced by SOS Battles, which were less useful for EV training and often made catching Pokémon needlessly frustrating.
• Sun and Moon brought back the Rustling Grass and Rippling Water phenomenons. However, unlike previous generation games, the ones seen here have quite a half-assed feel to them. You're introduced to the Rustling Grass mechanic early on to catch a rare encounter in Makuhita, but aside from that, Rustling Grass is pretty much non-existent, and the few times the phenomenon does occur ends up being for land Com Mons like Yungoos instead of being rare Pokémon. The Rippling Water phenomenons don't fare any better where instead of rare Pokémon, you'll often encounter water Com Mons like Tentacool and Wailmer.
• Regional forms are well-regarded for allowing fresh takes on classic Pokémon and potentially breathing new life into their designs and/or battle prowess. However, there's only ten lines of Pokémon with regional forms in the mechanic's debut generation and they're all Generation I Pokémon, resulting in disappointment for those who'd hoped for Pokemon from other regions to be revisited in this way. Later games addressed this by giving regional forms to Pokemon from other regions, though there's still a heavy bias towards Kanto mons.
• QR Code event Pokémon. Only one Pokémon was ever distributed in this format, Magearna, while all other distributions were done using the same methods as Generation VI, such as codes and local wireless at public events.
• Pokémon Let's Go, Pikachu! and Let's Go, Eevee! has an extremely brief gameplay section where the player takes direct control of their partner Pokémon to climb up the ductwork in the Team Rocket Hideout to retrieve an item. This is the only time in the entire game where such a mechanic is used, and its single use doesn't even allow the level of exploration you'd expect from the idea.

Gen VIII:

• The Escape Rope is now an unlimited-use Key Item instead of a consumable. This would have been amazingly convenient in a region with a lot of caves, like Johto, but Galar only has two or three "dungeon" areas it's usable in, and none of them are large enough that you're likely to need it. Furthermore, as the Escape Rope works by returning you to the overworld from a separate cave map, it's useless in the DLC areas because all the caves there are integrated into the overworld map.

Gen IX:

• Those who were excited to see new Paldean regional forms of older Pokémon ended up feeling a bit let down that only Tauros and the Wooper-line ended up getting new variants.
• The newly introduced convergent form gimmick, as an alternative to regional forms, also ended up being quite underutilized as it was done with just the Wiglett and Toedscool-lines. Two Pokémon families that are based on the Diglett and Tentacool-lines. Even when combined with regional forms, that's only four Pokémon families that received new Paldean variants. Three of which were, once again, from Gen I. Eventually, the Teal Mask DLC introduces the Poltchageist-line as a convergent family to the Sinistea-line, which gets rid of convergent forms being limited to just Gen I. However, that's still overall just five new variants.

Spin-offs

• The Pokémon Trading Card Game tends to have a lot of this:
  • This Hypno card has a Pokémon Power that lets it attack via an active Clefairy Doll, adding a new dimension to that card's mechanics. Not only is it the only card to have such an ability, it was a Japan-only promo.
  • For instance, Generation VI had Pokémon with Ancient Traits such as doubling how much health a Pokémon had restored, allowing you to attack two energy cards regularly and so on and Steam Siege had Dual-Type cards that hit an opponent for two types of damage such as Azumarill who is a Water/Fairy-type card and its Bubble Drain and Play Rough attacks hit the opponent for Water and Fairy-type damage.
  • A notable one is React Energy, a Special Energy Card that does nothing on its own and only provides one Colorless energy, but was used as a component in a number of attacks, Poké-Powers, and Poké-Bodies on various Pokémon. However, the only set it was ever used in was EX Legend Maker, with it never being used after that. The Team Plasma-based sets in Generation V would introduce Plasma Energy, which works the same as React Energy, but at least that got to be used in more than one set.