Bringing Pokemon to other regions that they are not in

[Greninjaman]

Just to clarify what I mean: I'm talking about taking a Pokemon that is mostly found in one region and placing it in another region. Like taking Houdour and putting it in Jagged Pass/Fiery Path area of Hoenn even though its not in the Hoenn Pokedex. Would it be weird if a Hoenn Gym Leader uses a Pokemon not in the Hoenn Pokedex (like Roxanne using Roggenrola even though they cannot be found in Hoenn [ignoring the fact that Roggenrola did not exist in Gen 3])?

 

[Welkamo]

I mean... people can travel and find Pokémon, trade across regional/international boundaries (depending on your worldbuilding, and going down that route has a lot of implications for a lot of different things)... heck, if you think a Pokémon would fit in a region that it doesn't appear in, go for it!

 

[Greninjaman]

It also would depend on the biomes of the region. Like, in a more tropical/warmer region, you would find more fire types then you would in a colder region.

 

[Steel_Justice]

The Pokemon anime does it all the time no matter the generation so it shouldn't be a problem. It often goes one step further and makes new areas to put them in. One that sticks out to me is how Ash and co encounter Houndour in Johto in one episode even though Houndour oddly don't appear in Johto pre HGSS.

From a game lore perspective there is the interesting case of Black and White 2. In the original BW you only find gen 5 Pokemon in most of the map but those same areas in BW2 will have mons from previous generations there. It could be for a couple of reasons. People do travel and sometimes bring Pokemon with them that are normally not in the region. Trainers could end up breeding non native Pokemon and end up releasing a ton of them into the wild on their quest for the perfect IVs. Migration could also be a factor for Flying and Water types.

And as for Gym Leaders + E4 they have had a history of having new mons when they appear in regions they aren't native to. The Johto and Kanto gym leaders have Hoenn and Sinnoh mons in their rematches in HGSS and everyone comes back with a vengeance in the Pokemon World Tournament in Black and White 2. I think if Jasmine can have a Metagross in her HGSS rematch then Roxanne having a Roggenrola is fine. It wouldn't be the craziest addition to her team since she has a Carracosta in one of her BW2 teams.

 

[Greninjaman]

There is also the fact that in "Let's Pikachu/Eevee", you can get the Alola Forms of Kanto Pokemon. So the idea that any Pokemon can appear anyway makes a lot of sense.

 

[JollyYoshi]

I would usually decide not to have significant characters use Pokémon not from their region. (I do admit this is purist me speaking) Though sometimes, there can be an interesting reason to why they have that Pokémon, and I think it’s worth it to let them have it!

 

[Kyrî]

Like everything else, it should serve to characterize the trainer who has that Pokémon. Yes. You can have a champion like Leon and give him a Dragonite and nobody would question it and it will be fine, but you can also offer context and say that he captured the Dragonite when he traveled to Johto and make him acquaintance with Clair. Or, using some anime lore, make it a gift he received from Lance when they met in their battle.

It would also vary from trainer to trainer. For example, nobody would question why a Champion, the E4 or some Gym Leader have a Pokémon from a different region since they're experienced trainers who probably travel and trade Pokemons with foreign trainers regularly. But for a rookie who just starts their journey, you can use the chance to write an interesting history explaining why they got the Pokémon. From their parents, maybe, or a mentor figure. Maybe the Pokémon in question managed to escape from some poachers. Maybe they found the PokeBall lost and you can have a subplot about the Pokémon missing their original trainer. Etc, etc.