With Nintendo still keeping very quiet with regards to releasing any solid information regarding the widely speculated upon Pokemon Z theories are flying as the fandom grasps at whatever straws are available whilst we wait for some official information.
It's been widely accepted for a while now that the focus of Pokémon Z would be the lesser legendary Zygarde introduced in Pokémon X and Y alongside Xerneas and Yveltal. You could've played through the previous games without encountering Zygarde if you skipped the Terminus Cave, so it's not exaggeration to say that it had a much lesser part in these games.
Recently leaked images from Japan's CoroCoro magazine (see above) appeared to show new forms of Zygarde, the Pokémon that the upcoming game will allegedly be concerned with. It's believed that Zygarde takes it's name from zygote - a cell formed through fertilisation. The zygote's genome contains all of the genetic information necessary to form a new individual, and is the starting point of the development of life.
Cell: Single cells that make up Zygarde, they have no consciousness of their own and are found scattered throughout the world.
Core: A self-aware core, essentially this is Zygarde's brain.
10% Form: A dog-like form that the core takes when it gathers 10% of it's cellular structure from absorbing nearby cells.
50% Form: A serpent-like form that the core takes when it gathers 50% of it's cellular structure from absorbing nearby cells. This is the legendary encountered by the player in the Terminus Cave in Pokémon X and Y.
Complete (100%) Form: A more humanoid form, this is the perfect form of Zygarde that the core takes when it gathers 100% of it's cells. Supposedly stronger than Xerneas and Yveltal combined. Woah.
The Norse Theory
There's already been much discussion about the relation between the most recent Pokémon generations and Norse mythology, with the generally accepted idea being that the Pokemon X and Y legendaries are based upon the creatures we find guarding the Norse World Tree - Yggdrasil. Yggdrasil has Midgard, the world of men, which is filled with stags (Xerneas) at the base and the blind eagle (Yvetal) who guards the realm of the Gods at the top of the tree.
Deep under the roots of Yggdrasil is trapped the serpent-dragon Niohoggr who guards the realm of the dead. This is clearly comparable with Zygarde who first appears in Pokémon X and Y as a snake like creature and, funnily enough, is found in a deep dark cave called Terminus: which means end destination.
Looking closely at the different forms of Zygarde, it does appear that there's a parallel between the various forms and Norse mythology again - this time linked to the offspring of the chaotic Norse god Loki.
Loki himself was a shape-shifter, and often used the changing of his form for trickery. He sired four children, three to the giantess (jötunn) Angrboða. These were the wolf Fenrir, the world-serpent Jörmungandr and the giantess Hel.
10% form: Fenrir
The monstrous wolf-God Fenrir is the son of Loki who is prophesied to kill Odin during the Norse apocalypse, Ragnarök. He grows at a tremendous pace and is eventually chained down by the Gods to halt his growth. (Perhaps a parallel with how Zygarde's shape evolves?)
Zygarde's 10% form appears in the shape of a canine, similar to the wolf-like Fenrir.
50% form: Jörmungandr (Midgard Serpent/World Serpent)
Jörmungandr was a giant serpent banished to Midgard (Earth) by Odin. It was large enough to wrap its entire body around the Earth and cause earthquakes by tightening its coils. Odin forced Jörmungandr to wrap himself around the planet and hold it forever, for if he let go the world would end.
Whilst it doesn't look long enough to wrap around the Earth, the 50% form of Zygarde is certainly serpent like in form and has fan like fins protruding from it's sides.
100% form: Hel
Loki's only daughter, Hel is a goddess who rules over Hel (the Norse version of hell, where the souls of those who died non-heroic deaths go). She has a gloomy, half dead appearance with flesh coloured half blue and half red, reflecting how she is half alive and half dead.
Unlike the other forms, the 100% form of Zygarde appears humanoid in structure and is - yep - blue coloured on one wing and red on the other.