Pokémon Brilliant Diamond and Shining Pearl are finally available on Nintendo Switch, giving fans the chance to return to the Sinnoh region. This isn't the only time that players will travel to Sinnoh on Nintendo Switch, as the upcoming Pokémon Legends: Arceus is set in an ancient version of the region, when it was known as Hisui. Pokémon Legends: Arceus will introduce Hisuian forms for Pokémon to the series, which raises the question of why they didn't appear in Pokémon Brilliant Diamond and Shining Pearl.
Pokémon Legends: Arceus is similar to Monster Hunter, in that it's going to be about individual missions, rather than a regular Pokémon journey. The player will live in one of the few small villages in the Hisui region, which is one of the only human outposts. The Hisui region is guarded by incredibly powerful Noble Pokémon, as well as all of the wild Pokémon in the region, which walk around freely and don't hide in tall grass. The player is going to be adventuring on a wild frontier in Pokémon Legends: Arceus and the harsh lands created harsh Pokémon.
Pokémon Legends: Arceus has Hisuian forms for existing Pokémon, as well as Hisuian evolutions. These include Kleavor, the Bug/Rock-type Pokémon evolution of Scyther, which has a body coated in stone and two axes for arms. There is also Basculegion, the Water/Ghost-type evolution of Basculin, which is haunted by the ghosts of those in its school that didn't survive the perilous waters of the Hisui region. Pokémon Legends: Arceus is introducing some awesome Pokémon to the series, but they could have appeared in Pokémon Brilliant Diamond and Shining Pearl first. There's a reason why the Hisuian forms aren't in Pokémon Brilliant Diamond and Shining Pearl, but it likely won't stay that way forever, and the ancient Pokémon may return in the modern era.
The Origins Of Hisuian Pokémon
The Pokémon series generally doesn't stress over retcons. If a new Pokémon type or evolution is introduced in the mainline series, then it's generally handwaved away, with everyone in the world accepting the new status-quo without hassle. In the case of regional forms, the explanation is that some Pokémon develop differently in some regions, due to adapting to a new climate. The Hisuian Pokémon that will be introduced in Pokémon Legends: Arceus resided in the region long before humans colonized the land. This explains why they are so different from their existing forms, as they had to survive in a much harsher environment. There are some Hisuian Pokémon that have dark backstories to reflect this, like how Pokémon Legends' Hisuian Zorua and Zoroark are vengeful ghosts, as they died after migrating to a new region, and have been reborn as instruments of revenge. The Hisuian Pokémon from Pokémon Legends: Arceus are products of a different time, which explains why they have never have appeared in the series until now.
Possible Reasons For Hisuian Pokémon Not Being In BDSP
There is an unfortunate implication to the absence of Hisuian Pokémon in the other Pokémon video games. The Hisui region was eventually taken over by humans, becoming the Sinnoh region that debuted in Pokémon Diamond and Pearl. This suggests that the Hisuian Pokémon died out in the transition. The exact nature of this extinction likely won't be brought up in the games, but there are a few different ways it could happen. Sharpedo's Pokédex entries mention how it was nearly hunted to extinction, as humans were eating the Pokémon for their fins. The presence of cities and pollution could have offed the Hisuian Pokémon, as they could not exist in the new environment. Extinction is also the backstory for many of the fossil Pokémon, as they died out long ago, and only exist in the modern era due to technology.
Pokémon Brilliant Diamond and Shining Pearl had the opportunity to include Hisuian Pokémon, in order to give fans a preview of them before their appearance in Pokémon Legends: Arceus, but they are nowhere to be found in the game. There is also no evidence of the Hisuian Pokémon in the files of the game, as Pokémon Brilliant Diamond and Shining Pearl leaked early, so fans were able to datamine everything that was present in the Sinnoh region before they were released. Pokémon Brilliant Diamond and Shining Pearl are faithful remakes of the original games, which is also a strong reason for why the Hisuian Pokémon never appeared, but all is not lost for the ancient Pokémon, as there is another notable absence in the games that could allow them to return in the future.
Legends Arceus Could Bring Hisuian Pokémon To BDSP
As of the time of writing, there is currently no way to encounter Arceus in Pokémon Brilliant Diamond and Shining Pearl. As a Mythical Pokémon, the player will need some kind of item from outside of the boundaries of the game to unlock it. There was actually a whole special battle with Arceus programmed into Diamond and Pearl, but the item needed for the event was never distributed. The item in question was the Azure Flute, which would create a stairway into the heavens, where the player would face Arceus in the Hall of Origin, and potentially catch it.
A recent trailer for Pokémon Legends: Arceus referenced the Azure Flute, as the instrument the player uses to call the rideable Pokémon makes the same noise when played. It's likely that the player will need to complete some event in Pokémon Legends: Arceus to unlock the Arceus encounter in Pokémon Brilliant Diamond and Shining Pearl. This suggests a direct connection between the two games, which makes sense, considering they are both set in the same region at different points in history. It's possible that Arceus will bring Hisuian Pokémon into Pokémon Brilliant Diamond and Shining Pearl, and players will be able to catch them in the wild.
The modern Pokémon games are able to add content in post-launch updates. This means Game Freak can add Hisuian forms to Pokémon Brilliant Diamond and Shining Pearl in a future patch. Pokémon Brilliant Diamond and Shining Pearl already have save unlock bonuses from existing Pokémon games on Nintendo Switch, so it's possible it could gain one from Pokémon Legends: Arceus, in order to encourage players to buy both games. This would be the perfect way to show the Hisuain Pokémon in a regular game, ahead of their formal introduction into the mainline entries in the series.