On today's episode of "speculation about Pokemon Z that may or may not have ever been in consideration"
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Ash-Greninja is a relic of scrapped Mega Kalos starters
Like my bit about Magearna originally being a gen 6 mon this is kinda just fueled off of circumstantial evidence so I'll just give the bulletpoints on how I believe things went down
-Pokemon Z was in development with Mega Kalos starters being part of it
-Anime staff were planning to do an arc like we got with Ash-Gren where Ash learned how to control Mega Evolution by bonding with Greninja and fighting in sync with it and yadda yadda yadda, basically the same story beats as the final product but with that mechanic
-Pokemon Z fell through and with that Mega Kalos starters died with it
-Anime staff couldn't/didn't want to scrap the planned Mega Greninja arc so they tweaked the design to be Ash specific and made it a form unique to him separate from Mega Evolution
Two things worth noting in addition to all this is that 1. It's weird how Kalos starters never got to join in on the fun of their gen's special battle mechanic when the Alola starters got exclusive Z-Moves and the Galar starters got Gigantamax forms which themselves were not in the base game proving GF is willing to wait a bit to give the starters special things from their gen and 2. It's likewise odd how Ash never used Mega Evolution in XYZ when he got to use Z-Moves and Gigantamax in their respective anime seasons
The biggest hole in this theory is probably Lycanroc-Dusk, another Ash-exclusive Pokemon form totally separate from Z-Moves and the lore surrounding them. Then again with Ash-Gren being a pretty big hit maybe they figured it wouldn't hurt to try that approach again