Over the past two generations, cover legendaries have been part dragon. It would be no surprise then, if the gen6 legends are part dragon. And with one of their legends being a forest Pokemon, this will add up to there being a grass/dragon typing.
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In my own opinion, I don't think that Xerneas is a dragon or a grass type. Sure, it may be a forest guardian, but it probably won't be a grass type. To me, it looks like a Psychic/Steel type. It could also be a pure Psychic type, but many possibilities are open for its typing.
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