Hi. I was wondering a bit about Greninja, and "Ash-Greninja", and thought about something. Well, what I was thinking was about Bonnie from the XY and the XY&Z anime called Ash's Greninja, the Greninja with Battle Bond, an "Ash-Greninja". And it's probably because of "Ash Greninja" having these black and red hair thingies representing Ash's red hat and his black hair. So that's why Bonnie named him like that. Because Greninja looks like Ash.
Then, the Pokémon was distributed in the game, The Pokémon Company had to find a name for it, they already had it, and probably only used that considering it is the only name what people know it for. Now, if you dare to say: 'But it's the first species so if you find it you can name itblajbljabjlhjblahblah' I remember there was in an episode with something about Greninja with special forms in older times, not sure, but you don't have a source either so hah. Also, it would be pretty weird if you changed Ash-Greninja to: "Greninja that is cool I think lolololololol xDDDDDDD". But if you use the name Ash-Greninja, would it be correcty-correctly-correct?