This Pokémon wearing a mask has been dubbed “Null,” meaning nothing.
The shapes of its front and hind legs are clearly different. The reason is that Type: Null was constructed to synthesize the strengths of various Pokémon, enabling it to adapt to any situation.
The mask fitted to Type: Null’s head is a piece of equipment designed to control its latent powers. It’s extremely heavy, so it also serves to hinder Type: Null’s agility.
To complete a certain mission, there was need of a Pokémon powerful enough to rival those Pokémon often spoken of in mythology.
This really makes it look like Pokémon X and Y are just demos for the functionality of the Pokémon Universe in a 3DS system and they went all out bonkers with Sun and Moon knowing the capabilities of expanding the lore of Pokémon.