Pokemon games from across all generations have had some creatures that are absurdly overpowered. Using them in your run-throughs of the game won't be problematic since you're only playing against a machine, and you can decide how challenging you want those battles to be.
However, bringing these Pokemon into a competitive setting is a whole other matter altogether. In tournament settings, the organizers have to make sure that the match is fair and that there aren't any absurdly powerful monsters skewing the outcome of the game. This is why some of them have to be banned from being used in competitive scenarios. Here is a list of ten such creatures and the reasons they were banned.
Mega Gengar, Mega Blaziken, and eight other overpowered Pocket Monsters that have been banned from competitive Pokemon at some point
1) Mega Gengar
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Mega Gengar's ability Cursed Body locked any creature that made contact with it using a physical attack on the battlefield, preventing it from switching out. Combined with access to the move Perish Song, it meant that the enemy Pokemon was sure to faint within three turns, while Gengar could save itself from that fate by switching out. This tactic was too overpowered, and it led to Mega Gengar getting banned from competitive matches.
2) Blaziken
Blaziken's hidden ability called Speed Boost makes it so that its speed is raised by one stage after the end of every turn. This meant that if Blaziken could avoid getting KO'd on the first one or two turns in any given matchup, it could outspeed everyone on the field and start knocking the opponent's Pokemon out with no chance of them retaliating. This was the reason behind Blaziken and its Mega form getting banned.
3) Mega Kangaskhan
Mega Kangaskhan is a force to be reckoned with since it sports 100+ points to all its stats except its Special Attack, which is basically useless, considering it can have its Attack stack doubled in two turns and stay on the field for longer with its massive bulk.
All this is thanks to its ability, Parental Bond, which turns all its attacks into two-strike attacks. This means the Parent Pokemon gets a double boost from attacks like Power-up Punch. This ability makes the creature too strong to be allowed in an official battle.
4) Mewtwo
There is not much to say about Mewtwo as there is no aspect in which Mewtwo is lacking. It has nearly 700 base stats, and it can learn all the moves that are competitively viable. Therefore it only makes sense that it is banned in formats where creatures like box legendaries are not allowed.
5) Arceus
The reason behind Arceus being banned is rather straightforward. Arceus is a literal god in the world of Pokemon. It is the source of all creation and the sole reason behind the existence of all the other Pocket Monsters in the world, so it would be more than unfair to pit them to fight against their creator.
6) Aegislash
Aegislash's ability to change forms between one that focuses on offense to one that is defensively built makes it possible for the player to have a variety of options to disrupt the opponent's game plan.
Combine that with nine resistances and three immunities thanks to its Ghost/Steel-typing, and it becomes a Pokemon that is nearly impossible to remove from the field, which is not conducive to a fair Pokemon battle.
7) Caterpie
Unlike the other creatures on this list, Caterpie is rather harmless to its fellow Pokemon. However, String Shot, a move it is seen using more often than not, is apparently so potent that it can break down the entire game, forcing organizers to restart matches when used. This is obviously caused by a bug (pun intended), but the developers deemed it better to ban the creature rather than fix the problem.
8) Ash Greninja
Ash Greninja, also known as Battle Bond Greninja, is an extremely fast, offensively leaning creature with a massive 640 base stat total. This is more than a lot of Legendary Pokemon and all of the regional pseudo-legendaries, who clock in at 600. Put all that together, and the creature becomes a menace that the developers decided was best avoided in competitive battles.
9) Porygon2
Porygon2 is only the second stage in a three-stage process of evolution. This means it can make use of the item Eviolite, which takes its defenses to 130+ stages without any EV investment or nature to help out. With those, it becomes a mass that is near impossible to remove from the field. Combine that with a potential moveset that includes Thunderbolt, Shadow Ball, Hyper Beam, Ice Beam, Psychic, and more, and it becomes a creature no one, including The Pokemon Company, wants to see on the competitive battlefield.
10) Togekiss
Togekiss is a fairly balanced creature. That is, until you look at its ability Serene Grace, which doubles the chances of secondary effects like Thunder Wave, Air Slash, Rock Slide, and so on. These moves have a crucial impact in competitive Pokemon battles, where every decision counts. This apparently slight edge that Togekiss gives its users got it banned from some formats.
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