Quoted By:
"Gen6: meganium and yanMEGA
Gen7: porygon Z"
If the secret of the starters aren't split forms, then what is it? Being called ash greninja, I wouldn't say it's not a possibility that it's a transformation, it could be related to synchro and bonds if it does involve z-moves (since, you know... Require crystals and bonding... At least I think). So the starters' secret could be this transformation/z-form (no confirmed name)