Greninja appears to have been added to Pokémon Brick Bronze in late 2016, with one guide video showing Ash-Greninja being added on December 20, 2016, and later gameplay videos featuring Greninja in March 2017.

Quick Scoop

  • Ash-Greninja was being discussed as an addition by December 20, 2016.
  • A gameplay episode titled “WE GOT GRENINJA !!!” was published on March 29, 2017, which shows Greninja was definitely in the game by then.
  • A wiki page later noted the shop “now sells Ash-Greninja,” but that page is much newer and doesn’t cleanly date the original addition.

Best estimate

The safest answer is: Greninja was added around December 2016, and was certainly present by March 2017.

Note

If you meant Ash-Greninja specifically , the clearest dated source points to December 20, 2016.