In the upcoming Pokémon Scarlet and Violet, the Legendary Pokémon may be different versions of the same Pokémon from two different time periods, Koraidon from the past and Miraidon from the future. This would fit well with the games' themes of past versus future, which are displayed with the Legendaries' designs and those of the twin Professors Sada and Turo. Scarlet and Violet are set to release on November 18, so there shouldn't be long before players discover whether this is true or not.
In Pokémon Scarlet and Violet, players will explore Paldea, a vast region that boasts a wide variety of different biomes. The games will be completely open-world with no Routes like in past games, and unlike the Hisui region, won't be divided into segments that players will have to unlock, making Pokémon SV the open world Legends: Arceus wasn't. Players will be aided in their journey by either Koraidon or Miraidon, depending on which version of the game they play. They'll be able to use them as Ride Pokémon to move swiftly over land, traverse open water, or glide through the air. Using Legendary Pokémon for such a purpose is quite surprising, but exciting at the same time.
Rumors surrounding Pokémon Scarlet and Violet have been around almost before their announcement, yet few have involved the Legendary Pokémon. The biggest ones were them being Ride Pokémon, which was proven true during the August 3rd Pokémon Presents video. Very little is known about them at the moment, so there's no evidence for or against the theory of them being the same Pokémon for now. Pokémon Scarlet and Violet's version-exclusive Legendaries wouldn't be the first to be linked together, however; Gen 7's Legendaries, Solgaleo and Lunala, are part of the same evolutionary line, which isn't exactly the same concept but does still set a precedent.
Are Scarlet & Violet's Koraidon & Miraidon The Same Pokémon?
The similarities between Koraidon and Miraidon are fairly obvious. Their appearances mimic each other in both their regular and vehicle forms, they have identical traversal modes that use incredibly similar methods involving their throats, tails, and the antenna/tendrils on their heads, and they serve the exact same purpose in-game, one that - presumably - no other Pokémon do. Even their names are almost identical, as there's only two letters separating them. Miraidon most certainly seems like it could be a version of Koraidon from a distant future, and Koraidon seems like it could be what Miraidon was before becoming a machine.
Rumors of "Regional Fakes" in Pokémon SV could also indicate that Koraidon and Miraidon are the same Pokémon. Paradox Forms are rumored alternate forms of returning Pokémon that have time traveled from the distant past and the far future, and while they haven't been confirmed, the information comes from trusted leakers such as Centro LEAKS, so the possibility looks promising. Paradox Forms are said to be classified as new Pokémon separate from their original forms, with new names and Pokédex numbers, which would fit with Koraidon and Miraidon being separate but the same. If these rumors are true, the theory becomes much more likely.
If the Legendaries are the same Pokémon, that brings into question what their present-day version is. It would presumably show up in some form in-game, and the best way could be as Pokémon SV's third Legendary. Since third Legendaries usually represent a force that acts as a balance for those of each of the Generations' box Legendaries, this would be very fitting. All of this could be a great plot twist, one that enhances Pokémon Scarlet and Violet's entire story, so hopefully the theory proves true.
Source: Centro LEAKS