It's actually a new type, one of two being introduced in X & Y. It's a light type.
It's so easy!
Huge stretch here and going off what I saw from the trailer. If X and Y are supposed to be a theme probably based around DNA and so a male/female thing for plot I'm guessing that the two legendaries are of the classification of Creation Power Pokemon.
We have the god of Pokemon universe Arceus who created everything after hatching out of the egg from a previous chaotic universe. Who were the parents? In this case I would like to believe that if we're getting another round of creation stories for this generation then Nintendo could introduce the X and Y legendaries as being the re-created parents of Arceus or the original parents. (Assuming Arceus requires parents) Perhaps the X/Y legendaries are the reason for a sex difference if there is any between certain Pokemon and so they formed each sex to look as they wanted them to look like.
Other things noted about Arceus is that he is every single type currently in the game and if they want to will probably create more type plates for Arceus to hold. We can only catch him (I'll just refer to it as a him) as a normal type but we know that's for balance and how the game data works but that would mean that in reality or how it would actually work is that he is only 1/17th (or how many other type there can be) of full strength. We also know that in the case of the three watchers Palkia, Dialga, and Giratina they can be recreated should a trainer ever catch them so that they can still do their duties to watch over space, time, and anti-matter.
He likely created Mew as a way to hold or to be the common ancestor to all past, current, and future Pokemon. We know Mew can learn every HM and TM in game so that all holds true.
most pokemon are 50/50 gender ratio. there are some exceptions both ways. eevee is a higher chance of being male and a grotto eevee is actually less than 1% chnce of being female. all starter pokemon except for in yellow are in the catagory of 12% chance of a female. there are very few that have that reversed. the pokemon with a higher female chance that i know of are skitty/delcatty, gothita and its 3 forms and vulpux/ninetails...beleive it or not mr.mime can be female. and unfortunatly there are male gardevoirs in the game.
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37 + (3*7) + (3*7)W/L/Death count: Wolf: 0/1/1 | Mafia: 1/6/7 | TPR: 0.5/4.5/5SK: 0/1/1 | VT: 1.5/3.5/5 | Cult: 1/0/1Glyphmaster Gunhaver | Avatar by Hasana-chan