Why at a stretch? If you look closely at its design and also look back to where it was in the trailer you will see it is actually very "grassy". Its antlers look like tree branches for example.
It was in the forest but Xerneas itself doesn't look grassy at all to me. Its antlers might look like tree branches but they're not brown, they're glowing with prismatic colors and look hard like steel more than wood.
Actually it is brown. http://i1.minus.com/iJeXdQxlpZ5EC.jpg
The antlers are part brown and shaped like tree branches with the other branches being rainbow coloured. Its legs are the same colour as the brown antlers as well, as is the small triangle thing on its chest.
This points to it being a sort of forest guardian/spirit, and given that it would likely be grass type. I'd also say it would be likely to be Fairy type as that is what fairies tend to be part of if the type ends up being real.
Official Piplup of the Pokemon X board. Current Location: In your nightmares with a knife and a gun and a chilling wail