Sure, there's pressure when coming up with new Pokemon for each main series installment, but what about the ones that appear on covers? That's an added bit of pressure, which Junichi Masuda discussed with Kotaku.

"It's definitely increasingly more difficult with each game to come up with the Pokemon that appear on the packaging. This time we had the theme of 'eternalness' with Xerneas and 'destruction' with Yveltal. We had these two themes, and coming with designs that really worked to represent these themes was really difficult."

Full interview here