Maybe you or someone saw
this article and it made connections with the starters because they talk about them at first.
But it sounds like it's just sakurai and his team that decide.
"Sakurai
planned on including a Pokémon from the then-latest generation. He discussed the matter with his team, and they eventually reached a consensus on Greninja. When Sakurai was made aware of it, all he had to work with were reference drawings. But that was enough to inspire him; that same day, Sakurai stayed awake until midnight to conceptualize Greninja. "
"Picking up on what we were getting at, Sakurai explained that the development team wasn’t waiting to pull something out of
Pokémon X and
Pokémon Y—there was an open slot for a Pokémon, and the team began discussing who to put in it. Eventually, after a conversation among the various staff, they settled upon Greninja."