Looks like our club grown since we have now got 15 members (I counted correct me if I'm wrong)

About a water type legendary. Now the mascots of Pokemon X and Y are based off Norse Mythology. The main symbolised creatures are a Deer, a wvyern like creature and a serpent

Now Xerneas is a deer, while Yveltal is a flying creature which I think is based off the wvyern. But there is the serpent

There are many ways a serpent can be applied into the game. It could be another Dragon noting the link to Dragons and reptiles or it could be Poison type due to how Snakes which are serpents use poison to kill. There is the possibility of the Water noting the ideas of sea-serpents, so who knows X and Y are incomplete without a Z, so anything could happen there may be a Pokemon Z with a Water type Serpent pokemon as the mascot.