Fandom! It's just confusing because Ash does get Mijumaru. So why even mention Meguroko then? This is why I hate making threads. People don't know what's real or fake anymore from the stuff that has been confirmed and the stuff that's not.
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I really think you're confused. As I said the title of the episode has no bearing on who Ash catches.
Just because he catches Mijumaru does NOT mean he will catch Meguroko.
Why mention Meguroko then?
Why the heck ask us? we're not the ones who created the title, The people who creates the anime created the title. Not us.
But my best guess is because Meguroko is actually going to be in the episode, that's why it's in the title. NOT because Ash will catch it but because it will be in the episode. Likewise Mijumaru as well will be in the title. Will Ash catch it? It depends on entirely on 2 things.
1. Is this Mijumaru's capture episode or could BW002 be Mijumaru's capture episode?
2. Is this Mijumaru Ash's Mijumaru?
And hypothetically speaking if the Mijumaru in the title is Ash's and this is Mijumaru's capture episode. MAKE note that it has no bearing on Meguroko in the episode whatsoever. For all we know Meguroko could belong to another character that might show up in the episode.
So again the best reason why Meguroko is in the title is because it will be in the episode. Nothing more, nothing less.