A picture from this months Corocoro Comic has leaked. The resolution is quite low on the picture but it is quite easy to see Arceus
. Arceus is the 493rd Pokemon and it's type changes based on any attached Plate items. It was announced in last months issue of Corocoro that a new Pokemon would be revealed in this issue.
Arceus has been know to the fan community since Diamond and Pearl was released in Japan, but it had not been officially revealed. For more information on Arceus, you can visit the dex.