The world of Pokémon has no shortage of captivating mysteries that have left fans digging into every ounce of lore left behind by Game Freak. Recent games have spawned many fan theories, especially as the series has begun dipping into some pretty wild storylines. While Pokémon Scarlet and Violet's time travel-infused storyline has already created a ton of speculation regarding future DLC, another recent Pokémon game has a mystery that's a bit easier to overlook.
In Pokémon Legends: Arceus, players explore the ancient history of the Sinnoh region called Hisui. There are many regional variants in Hisui, including new forms of Voltorb, Zoroark, Stantler, and Growlithe, among others. Obviously, these new Hisui Pokémon couldn't be included in previous games as they didn't conceptually exist yet. However, the in-lore reason why these Hisui Pokémon have seemingly gone extinct has more than captivated fans.
Hisui Pokémon Don't Exist in Paldea (Yet)
It's currently not possible to obtain any Hisui Pokémon in Pokémon Scarlet and Violet. However, fans have found data for every regional form in the game's code, which all but confirms that they'll at least be obtainable through Pokémon Home in the future. So, eventually, trainers will be able to use their Hisui Pokémon from Arceus in Scarlet and Violet. However, what's kept fans speculating is how there's almost no reference to any Hisui regional forms throughout the Generation 9 games.
From an in-game lore perspective, it's almost as if Hisui's unique Pokémon forms simply vanished at some point between Arceus and the modern day. Fan theories range from things like the process of biological evolution to the Pokémon being hunted down to extinction by humans. While nothing's been fully confirmed by Game Freak at the time of writing, there is the possibility that DLC to Scarlet and Violet may answer these questions. For now, there are many interesting fan theories that may just provide an answer.
Overhunting Might Have Forced Hisui Pokémon to Adapt
Popularized by The Game Theorists on YouTube, there's a solid argument to be made that many of Hisui's regional Pokémon were forced to adapt as the humans began to hunt them for resources. A great example comes with Wyrdeer, Stantler's Hisui-exclusive evolution. Wyrdeer's Pokédex entry in Arceus specifically notes that humans would frequently hunt the Pokémon for its fur. This also explains why Stantler can no longer learn Shield Bash, as using that move is what allowed it to evolve in Hisui.
Hisuian Voltorb also adds credence to this theory. Hisuian Voltorb's Pokédex entry notes that it seems to be made out of a very similar material to Apricorns, which are a crucial material for crafting Pokéballs in Arceus. It's also noteworthy that Apricorn trees are not present in Sinnoh in the modern era, despite being fairly abundant in Arceus. Piecing these two oddities together, it's believable that Voltorbs were hunted for their Apricorn-like bodies.
Hisui's Regional Variants Might Have Just Adapted to the Changing Times
A far less depressing theory proposes that humans had a far less direct role in the disappearance of Hisui Pokémon. Rather than being hunted down, the Hisui Pokémon might have simply changed along with the technology of the time. Hisuian Voltorb, for example, used its wooden appearance to blend in as a defense mechanism. However, as humans moved from using wooden Pokéballs to metal ones, Voltorb could have simply adapted its appearance to better fit with the times.
Similarly, Hisuian Zoroark might have changed its snow-white coat as it moved away from Hisui's colder mountains and into other regions. Given that Zoroark has always been about illusions and disguises, it makes sense that it would try to blend into its environment. While this theory involves far fewer examples of Pokémon being hunted by humans, it doesn't explain every Hisuian Pokémon. Stantler forgetting the move Shield Bash, for example, doesn't really have an explanation that fits with this theory.
The Hisui Pokémon Might Have Migrated as the World Changed
One final, smaller theory that was proposed by several fans is that Hisui Pokémon actually migrated away from the region. Rather than adapting or changing, they simply left Hisui as rapid industrialization changed the region. There's some evidence to back this up, such as with the Arceus website establishing that normal Zorua had migrated to the Hisui region before they became their regional variants.
Migrating between regions has been mentioned in several Pokédex entries in the past, so it's entirely likely that the Hisuian variants simply left Hisui. This leaves the door open to them showing up in a new region down the line and is probably the simplest explanation as to why Hisui Pokémon seemingly vanished. However, there isn't anything that directly supports this theory outside the previously discussed in-game examples of Pokémon migrating.