ZSquishy
Zygarde is a green and black Pokémon composed of smaller units known as Zygarde Cores and Zygarde Cells, who collectively define its appearance and power by assembling in different percentages. In all of Zygarde's forms, the various Cores and Cells are represented as green and black hexagons that glow when it attacks. It has also been shown to be able to fly in all of its forms.
Zygarde is the guardian of the Pokémon world's ecosystem, and is responsible for monitoring it for abnormalities from caves such as Kalos's Terminus Cave and Alola's Resolution Cave. It is capable of reversing the effects of 's Fairy Aura and 's Dark Aura, thus ensuring the equilibrium of the cycle of life and death are kept in check. Normally, Zygarde avoids getting involved in conflicts unless the ecosystem is in grave danger. It is also known to ask for humans to assist it in protecting the ecosystem when necessary, but can still be wary of those it does not trust. In the Pokémon Adventures manga and in Pokémon the Series, it has been referred to as "Z". In the animated series, a Zygarde Core and two Zygarde in their 50% Forme have had the green coloration on their bodies changed to red as a result of the influence of Team Flare's Mega Evolution energy weapon; moreover, all of their formes were depicted as capable of human speech via telepathy.
Zygarde Cores and Zygarde Cells
Physically, a Zygarde Core and a Zygarde Cell are almost identical. Both are small, green, cell-like blobs that resemble a planarian. Their bodies are gelatinous, semitranslucent, and can be segmented into three sections: a large, round head, an abdomen with a colored hexagon, and a stubby tail. They have two eyes, though one is larger and has a pupil, and can stand on their tail (a trait most associated with the Core). The difference between a Core and a Cell is subtle: Cores have a black pupil and a red hexagon embedded inside their abdomen, while these are both green on Cells.
On their own, neither Cores nor Cells have the power to battle, needing to assemble in order to take action. The Cells exist in far greater numbers than the Cores and, therefore, make up the majority of any given form of Zygarde. However, they lack wills of their own and can only act in accordance to the orders of a Core. The Cores act as Zygarde's brain, and Zygarde may have up to five of them. It can also use its Cells for attacks, such as raining them down like arrows or scattering them throughout the ground in waves before they're recalled into its body. When Zygarde exhausts all of its power, it splits back into individual Cores and Cells and scatters all over the world, reaching even far-off lands. While in this state, they absorb the native power of said lands and may request assistance from humans in order to reassemble. As seen in various media, the Zygarde Cells demonstrated some form of will of their own and are capable of moving independently, without Zygarde Core commanding them.
Forms
Zygarde has three forms: 10% Forme, 50% Forme, and Complete Forme, which are composed of Zygarde Cores and Zygarde Cells. Zygarde Complete Forme can Mega Evolve into Mega Zygarde.
10% Forme
Zygarde 10% Forme is a relatively small canine Pokémon. Its body is primarily black, though its neck, muzzle, mouth, eyebrows, and left front paw are green. Additionally, it has two green hexagons on its chest and a single one on its rear. From its neck, a green ribbon-like extension comes out, next to an elongated red hexagon, giving the impression of a leash and dog tag, respectively. Its eyes are white hexagons with no discernible pupils, and there is a thin white line on its forehead. Its mouth is fitted with razor-sharp fangs. On each of its legs are black hexagonal extensions: two on the back of each elbow, one behind each knee, and two on each hip. One of the hip extensions is longer than the other on each hip.
Zygarde's 10% Forme is is assumed in cases of emergency. It is created when only a tenth of its Cells have gathered, and is the result of its Cores calling Cells across the land when it needs to fight. It is skilled at close-range combat, overwhelming its opponents by leaping at their chests and sinking its sharp fangs into them. It is also capable of running at speeds greater than 60 mph (100 km/h). However, it cannot maintain this form indefinitely, and it will eventually fall apart. Concept art of Zygarde shows that it can wrap its "leash" around the arm of someone it trusts.
50% Forme
Zygarde 50% Forme is a large serpentine Pokémon. Most of its back, head, and neck are black, while the underbelly and the tip of its tail are green. Its eyes are made of four green hexagons, similar to an insect with compound eyes, and there is a thin, white line down the center of its face. Across the back of its neck is a large frill made of five long, flat protrusions that each have two small, green hexagons on them. There are three short tendrils on either side of its chest; they are black on the right and green on the left. Its tail also ends in five tendrils. It has several small, green hexagons on its body that occasionally glow white.
Zygarde 50% Forme is assembled from half of its Cores and Cells reuniting. Despite only using half of its full potential, its power is capable of overwhelming anyone who stands in its way. The 50% Forme is known to be both ruthless and merciless. There are rumors that an even greater power lies hidden within it, being capable of changing to a more powerful form when battling those who threaten the ecosystem. Its frill can be shown vibrating as it attacks. As seen in Down to the Fiery Finish! (XY131), Zygarde 50% Forme can summon and control massive roots for attacks and covering areas.
Complete Forme
Zygarde Complete Form is a large, humanoid Pokémon. There are several flat protrusions around its head and triangular protrusions around its neck. These protrusions create a crown shape. On its face are three white hexagonal marks: one in the middle of its forehead with a line through it and two that act as eyes. There are an additional five green hexagons on its face: four along the bottom and one in the middle of a short protrusion on top. Each shoulder has two serpentine wings protruding from it. The wings all have an undulating pattern and a hexagon near the tip: blue on the right side and red on the left. There are multiple white protrusions on its body: three on each hand acting as claws, one on each knee, and one and the front of each foot. Additionally, there are green hexagons surrounding its wrists and two under the white protrusions on its knees and feet. Its abdomen has a vertical, hexagonal pattern with a thin, white line on either side. Across its chest is a thin, zigzagging mouth that when opened, reveals five hexagons colored red, orange, white, cyan, and blue, respectively. It has a long tail similar to Zygarde 50% Forme's body.
The Complete Forme is Zygarde's perfected form, created when 100% of its Cells and Cores are gathered. In this state, Zygarde can fight at full power and complete its duty of protecting the ecosystem by suppressing those who willingly endanger it. Zygarde Complete Forme contains separate Cores for each of its wings and its tail. In this case, each Core can make decisions and act independently of the others. The Zygarde Cores can freely change the shapes of the four wings and tail for a variety of roles. Notably, the tip of its wings can form snake-like heads that can be used to attack with energy beams, propel itself, and restore an unbalanced ecosystem back to normal, as seen in Forming a More Perfect Union! (XY136). The zigzagging mouth on its chest can radiate enough high-powered energy to destroy everything around it, even firing energy beams from within it. It has also been shown to form a green, protective barrier to shield itself and its allies. It has also been shown using Core Enforcer as a rush attack, leaving a Z-shaped carving in its opponents while dashing through them before it explodes. The Complete Forme is only used to neutralize huge threats to the ecosystem, such as 's Giant Rock, the ultimate weapon, Ultra Necrozma, and Hyperrogue Ange Floette. It is powerful enough to overwhelm even Xerneas or Yveltal.
Mega Zygarde
Overall, Mega Zygarde retains most of Zygarde Complete Forme's humanoid structure, but the Cells composing its body have been strategically relocated. Notably, it has lost most, if not all, of the hexagons composing Zygarde Complete Forme's lower body, arms, and wings, exposing its light green body. As a result of this, Mega Zygarde has completely lost its wings, having seemingly replaced them with six smaller arm-like structures. On its legs, only the three hexagons on each knee remain, though they changed from symmetrical white and green to asymmetrical black and white. The lower part of Zygarde Complete Forme's chest, starting from the multicolored mouth, has also opened, giving the impression it has ripped into strands of black hexagons similar to tree roots. The green, eye-like patterns on its chest now have each a differently colored hexagon on them (one red, one blue). Underneath the chest plate, a series of five white hexagons cover the true body. White, chain-like hexagonal patterns run down Mega Zygarde's uncovered torso, arms, and legs, as well as on its back and under its feet. Its arms don't end in hands, but split into three hexagonal digits. Mega Zygarde's headpiece and tail remain mostly unchanged from its base form, only having lost the green hexagons on its face and having more of those on its tail. Its crown is lit up by a green flame, and its eyes are green as well.
Mega Zygarde is said to be born as a result of people's emotions during an unprecedented crisis, allowing it to calm the situation with its unmatched power. Its defining feature is the giant cannon held above Mega Zygarde's head by its six smaller arms. The cannon has a cylindrical shape, somewhat resembling a tree trunk, decorated with several shapes and lines similar to Zygarde 10% Forme's leash. The cannon is mostly black, with dark blue and dark red patches circling its length. At the cannon's muzzle, there is a pink symbol resembling a cross between an "X" and a "Y" surrounded by red, blue, and pink lines. Its interior features a circle, three hexagons, and a central circle, while its back has a dark green hexagon at the center. When Mega Zygarde attacks with its cannon, the muzzle pattern shines in the shape of an "X", "Y", and "Z" before opening up, unleashing a beam of light that defies nature itself, which has been likened to the Z-Moves found in the Alola region. Such an attack was powerful enough to overpower Hyperrogue Ange Floette's powered-up Light of Ruin, which threatened to annihilate all of Lumiose City.
(Source: Bulbapedia)