Before the release of the full version of Pokémon Sun and Moon, players could download a demo to get used to the new interface and mechanics in the game, and this also awarded the participants with a special Greninja that can turn into an Ash-Greninja upon knocking out one Pokémon. This form increases its stats and makes the move Water Shuriken more powerful.
To get it, you need to have the demo downloaded, as well as a physical or digital version of either Sun or Moon. Once you have it, boot up the demo from the 3DS and speak with professor Kukui. His dialogue options include "Bring to full version". Now you can send any special items you found, and the Greninja to the full version of the game. Now you can exit the demo and start the retail game. Make sure you have an open slot on your team, and head to the nearest Pokémon Center and talk with the man in the yellow and blue shirt near the desk. Accept his Mystery Gift, and you will receive the special Greninja from the demo.