This rumor is about the "100%" or the "complete" form of Zygarde, shown here:
This rumor deals with the red and blue we see on it's chest and one the "wing-like" parts floating above the arms. It is rumored that it may be possible that X amount of cells make the Core Zygarde, and more makes it the 10% version, and more makes to 50% the original Zygarde we are all familiar with. But the Complete version may expand upon this as to reach it you may have to fuse the 50% Zygarde with Xerneas and Yveltal, the three shown here:
As you can tell Xerneas has a Blue makeup to it while Yveltal had a Red makeup to it. Therefore this rumor states that because of the Red and Blue shown only in the Complete form of Zygarde that to reach this you may have to fuse these three legendarys.
This leads onto a subculture of Pokemon fans, something that has never been a part of Pokemon officially but Pokefans have gotten much joy out of: Pokemon fusion. This was first inspired by Alexonslager's website simply called Pokemon Fusion. What you do is you go to his website and you can "fuse" two Pokemon together. You can randomize the whole thing or one Pokemon at a time, or you can pick from a list. Check it out here: http://pokemon.alexonsager.net/
It is pretty fun, isn't it? Well it goes beyond that website and has lead people to draw some of these fusions. You can check out some artwork done by some awesome fans here: http://www.buzzfeed.com/awesomer/30-incredible-works-of-art-inspired-by-pokemon-fusion#.ov8KVDOpP
Pretty cool, right? Well what if in the Pokemon games we could fuse two Pokemon together?! Wouldn't that be awesome?! Like Cubone and Charizard:
So with this thought mixed with the possibility of fusing together Zygarde with Xerneas and Yveltal to get Zygarde's Complete form, maybe there is some hope that Pokemon Fusion will come to the games, it would certainly revamp the game and probably attract new and old players alike, but with all the new Mega evolution stuff, maybe they will hold off on something so groundbreaking.