My artwork for Zygarde, the Dragon/Ground-Type "Z" Legendary known as the Order Pokémon from Pokémon X and Y. As soon as it was leaked last night I started drawing it. Had this art finished at 3 a.m. last night but could't get back online to upload it. Based on the images I saw, it seems some of the hexagon-shaped "scale" parts glow or sporadically blink white, so I reflected that in the artwork here. I'm kinda pleased with how this turned out.
Edit: Added the shiny version.
On the other hand, it's because it's so obvious I think it's unlikely. Remember how sure everyone was that we were getting Pokemon Gray in Gen V? The fact that there's only three legendaries also seems suspicious. I think after everything that's been revealed early because someone found it in the game data (Darkrai, Shaymin, Arceus, etc.), any Gen VI hidden legendaries, possibly including the third version mascot, just aren't in the data for X and Y.
Of course, even if it isn't the last version mascot, that doesn't mean it won't get a new form. Where did you find this indigo serpent form? I couldn't find it.