With the story the game has (with a Legendary Dragon Pokemon splitting into Zekrom and Reshiram and leaving Kyurem behind) this is not surprising. Kyurem supposedly represents Wuji, the absence of Yin and Yang (which Zekrom and Reshiram represent). His alternate formes are simply him reclaiming one of his halves.
Cover art is pretty badass though.
Cover art is pretty badass though.