The oficcial webpage for the 12th movie is updated and the pichu that was mention below appears in the new trailler here is the adress: http://www.pokemon-movie.jp/ 

here is the trailer and by the end of it its state perfectly clear that pichu is the pokemon behind the question mark.¬¬ 

The official sites have updated with new Movie 12 information. Apart from confirming the title yesterday, it confirms that the movie concludes the DP trilogy and will explain why Dialga & Palkia are fighting and their feud with Giratina. It also confirmed details about the special Pichu. This Pichu is specifically Female, and is called a Notched-Ear Pichu, due to its left-ear being different, and it seems to enjoy the Grepa Berry. This may mean more gender differences are coming, but thats all the information thats known now. More information will come next month. 

and this is the poster of the movie, curiously when you save the image the name of the document is 12_god ¬¬