Xerneas is most likely based on the stag from Princess Mononoke.
It's probably not based on a (while admittedly popular) movie like that, most Pokémon draw from real world cultures and animals/plants/objects and legendaries in particular usually draw from mythology or folklore.
From this stand point, this is an incredibly unbalanced rivalry between the two legends. The game hasnt even come out yet, and xerneas already has an upper hand. We still dont know if fairies are super effective against dark or flying. Also, why give a new type to one legend but not the other?