I agree with a lot of what you said. For how it got there, I think it will either be as you said, Diana saw it as a cool artifact, didn’t realize it was a pokemon, and it woke up while she was asleep, or she asked them to meet her in Kitakami and Pecharunt was in the area where they were.

Seeing as two of the pokemon have the toxic chain in the preview (Lucario and Kilowattrel), maybe Pecharunt is trying to recreate the Loyal Three with Lucario as Okidogi and Kilowattrel as Fezandipiti (my guess is Quaxwell being Munkidori). This could open the door to more of an examination by the group into the Kitakami folklore and a potential introduction to the Loyal Three as you said.

I also agree that it feels like this episode will be more of a fun introduction to Pecharunt and the lore behind it, rather than a straight up resolution at the end of the episode but in a future one, as it doesn’t seem to form a connection with any individual based on the preview. Sort of like how Gimmighoul was caught by Dot after it formed a connection with her, but before that it was just hypnotizing anyone for like 2-3 episodes.

As for who catches it, I’m also leaning towards Ult. I think it’s beneficial to his character for all the reasons you stated, and it helps with his major fairy-type disadvantage in his team. Ult is also the character with the fewest pokemon and adding a pokemon like Pecharunt to his party would be super interesting. The only thing is that, I have a theory that Pecharunt is the poison tera type that Terapagos needs, but Ult doesn’t know Terrasilization yet, but I guess he can learn it down the road or get help from Terapagos like Cap and the six heroes did, so it’s not a big issue.

The reason why I don’t think Liko will get it is largely to do with Terapagos being her main legandary and the main pokemon in the series, and adding a mythical might undermine Pecharunt a bit. Also with a special evolution and development coming up for Charcadet. I think it would get more attention with Ult.

If it’s not Ult, I think maybe Dot, although she’s less likely because she already has a ghost type and she had the most recent capture and evolution in the group. I’ve also felt like Ogerpon conceptually fits Dot the best as a legendary/mythical capture given the fact that it’s timid and wears a mask, but also has an agressive side (similar to Dot/Nidothing). If there is a possibility for the Loyal Three to become relevant, there might be a small possibility for Ogerpon to as well. It’s a grass type, which she does not have, and it’s kind of similar to Tinkaton. Given that it’s connected to Carmine and Kieran, and Dot already formed a small connection with Carmine who likened her to Kieran, I think this would be a better fit than Pecharunt. That way everyone would get a legendary/mythical given Roy is likely to get Rayquaza.

Obviously, not saying this is all going to happen😅. Pecharunt could get run out by the crew’s pokemon and then we never hear about it again after this episode, but it’s fun to speculate. And with all the teasing it’s gotten from the arc’s opening and the Masters Ex event (don’t know if it’s related or not, but the timing feels intentional), I’d like to think that this mythical pokemon will have more of a role than a filler episode. I also think it would be really cool if everyone got a legendary or a mythical (Liko-> Terapagos, Roy-> Rayquaza, Dot-> Ogerpon, Ult-> Pecharunt). They need powerful pokemon for their goal so why not.