While it is not known to evolve into or from any other Pokémon, it has the following three Formes, which all retain Zygarde's Dragon/Ground typing:
Zygarde 10% Forme, one of the Formes introduced in Sun and Moon
Zygarde 50% Forme, the Forme introduced in X and Y
Zygarde Complete Forme, one of the Formes introduced in Sun and Moon
Along with these Formes are two basic components of Zygarde: Zygarde Cores, which act as the brains of Zygarde, and Zygarde Cells, which compose Zygarde's Formes and cannot think. Neither Zygarde Cores nor Zygarde Cells can use moves or are useful in battles by themselves. Both of these components were introduced in Pokémon Sun and Moon. They can be collected using the Zygarde Cube.
Zygarde 50% Forme was introduced in Pokémon X and Y. Zygarde Cell, Zygarde Core, Zygarde 10% Forme, and Zygarde Complete Forme were announced in the October 2015 issue of CoroCoro magazine. The remaining forms were introduced in Pokémon Sun and Moon. Prior to their appearance in Sun and Moon, Zygarde's other forms played an important role in the anime's XY&Z arc.
Zygarde 50% Forme has a serpentine body with green and black markings. Most of its back is 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. Across the back of its neck is a large frill made of five long, flat protrusions. There are three short tendrils on either side of its chest, and its tail ends in five tendrils. It has many small, green hexagon markings on its body that can glow. This form of Zygarde is thought to monitor those who destroy the ecosystem from deep with its cave. Should the region's ecosystem fall into disarray, it will appear and reveal its power.
Zygarde 10% Forme has a relatively small, mostly black canine body with a green "leash" that continues beyond the neck. Its muzzle is black at the top and green elsewhere. Its eyes are white hexagons with a black outline and no discernible pupils; directly above the eyes are two small green hexagons that resemble eyebrows. On its forehead is a very thin, white ellipse. Its "leash" is entirely green with a small red hexagon connected to the bottom. Its left front paw is green up to the elbow, while all its other legs are completely black. On each of its legs are hexagonal protrusions, all the same color black as the legs themselves. There are two such protrusions on the back of its elbows, one on each knee, and two on each hip. One extension on the right hip is placed higher than on the left.
Zygarde Complete Forme has a large, humanoid body with a long tail similar to Zygarde 50% Forme's body. There are several flat protrusions around its head and triangular protrusions around its neck, giving it a crown-shaped appearance. On its face are three white hexagonal marks: one in the middle of its forehead with a line through it and two being that acting 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 wings protruding from it. These wings all have a hexagon and an undulating pattern: blue on the right side and red on the left. The very tip of the wings can open up, making them appear to be a pair of "mouths" with red or blue patterns on the inside. It has 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 extensions surrounding its wrists and two under the white protrusions on its knees and feet. Its abdomen has a vertical, hexagonal pattern upon it with two thin, white, and vertical marks on either side. Across its chest is thin, zigzagging opening that fades between five colors: red, orange, white, cyan, and blue. The chest can open, revealing five cores of each color. Above this opening are two large green, hexagonal marks. When its wings and tail are extended, Zygarde Complete Forme resembles the Latin alphabet letter Z.
Zygarde Core is the brain of Zygarde, and has a small and leafy-green gelatinous body comprised of a large, round head, a moderately sized body, and a thin, hook-shaped bottom section that may be a tail. Its head is rounded except for a protruding, beak-shaped area at the bottom front. Within its head is a single, oval eye that has a smaller, oval pupil within it. Beside this eye is another white, semi-elliptic shape, possibly a second eye. Within its midsection is a nearly regular, red hexagon. Zygarde Cells make up every other part of Zygarde, and appears very similar to the Zygarde Core. Unlike the Core, it is flat, has a green pupil, and has a darker green, more elongated hexagon within its midsection. Unlike the Cores' hook-like bottom section, the Cells have a long and more noticeably tail-like back-end.
Zygarde Core art
Zygarde Cell art
Each of Zygarde's Formes is made up of Cores and Cells. Neither of these two forms have power to battle alone, only when combined to form Zygarde. The Cores have their own will and are sometimes able to act on their own. They serve as brains for Zygarde. The Cells form the rest of Zygarde's body. They are only able to act on the Core's orders. A single Core acts as the brain for the 10% form. This allows it to take action separately from Zygarde's 50% Forme, which comprises its main body, but only for a short time. Zygarde Complete Forme contains separate Cores for each of its wings and its tail, and each Core can make decisions and act independently of the others.
Each form of Zygarde has its own special abilities and methods of attack. The Cores are able to communicate with other Cores and Cells via telepathy. The Cores have been known to take action when the local ecosystem is threatened. Its 10% Forme has razor-sharp teeth and can reach speeds up to 60 mph (100 km/h). Zygarde Complete Forme is said to exceed both Xerneas and Yveltal in power, possessing 100% of its total cells. Zygarde only enters Complete Forme when the ecosystem is under threat and the Cores decide that 50% Zygarde is unable to deal with it.
Squishy later transformed into its 10% Forme after being separated from Bonnie and the others to defend itself against Bryony and her subordinates; Squishy, however, eventually returned to travel with Bonnie in A Cellular Connection!.
Zygarde debuts in Hawlucha Attack. It was first mentioned by Blue, who aimed to find it in order to stop the endless battle between Xerneas and Yveltal. It appeared in person at Anistar City, where it was sought after by Essentia, who aimed to capture it for Team Flare. When X and Y battle Essentia in order to retrieve Y's stolen Xerxer, Zygarde appeared before them and attacked before Essentia manages to successfully capture it.
Zygarde is later used by Essentia to battle against X, Blue, and Blaine. After Emma is freed from Lysandre's control, she was about to order Zygarde to attack him but he manages to take its Poké Ball making it fully under his control. Zygarde is later used to fight against X's Mega Evolved team, but as the battle progresses Emma notices green blobs falling off of it. At that moment, Xerneas and Yveltal are returning into their dormant forms, causing it to disintegrate, and X hits it with a combined attack from his team. This resulted in Zygarde exploding into millions of green blobs, and Lysandre begins to fall. Soon after the battle ended, Blue and Blaine visited Sycamore and mentioned to him about seeing a little green silhouette after Zygarde disintegrated into the blobs. Sycamore reveals to them that similar green silhouettes, which he dubs Zygarde Cells, have been spotted in a far away region as well, and sends Dexio and Sina to investigate them in Alola.
Zygarde 10% Forme may be based on Fenrir. The green leash-like object around its neck could be based on Gleipnir, the rope that binds Fenrir. This Forme could also draw inspiration from Garmr, the watchdog that guards Helheim's gate. In appearance, Zygarde 10% Forme resembles a Doberman Pinscher.
Zygarde 50% Forme's appearance is based on a serpent in the shape of the letter Z. It may also draw inspiration from Jörmungandr, a sea serpent that encircles the earth, and Níðhöggr, a dragon that gnaws at the roots of Yggdrasill, the world tree. Its flat shape and wriggling movement may be inspired by the tsuchinoko, a Japanese cryptid, and its snakelike appearance may be inspired by nāga, a class of snake entities in South and Southeast Asian cultures.
Zygarde Complete Forme may be based on Hel, the ruler of Helheim, the realm of the dead in Nordic myth. The colors on Complete Forme's chest match the colors of the real-world flag of France, the country which inspired the region of Kalos. These colors also match the colors Xerneas and Yveltal. Design-wise, Zygarde Complete Forme resembles a humanoid giant mecha despite its organic basis; its light-up technological appearance, "transforming"/"combining" gimmick and the face-like pattern on its chest are all tropes commonly used for fictional mecha.
Zygarde Cell and Zygarde Core may be based on cells as well as single-celled organisms such as flatworms (specifically planaria), slime molds and members of Euglena. Some planarians' ocelli also resemble eyes, which may have inspired the designs for the Cell and Core Zygarde. The hexagons within them may represent a cell nucleus.
Zygarde comes from the letter Z. It may also be derived from ζυγωτός zygōtos (Greek for joined), zygote, garde (French for guard), midgard (Old Norse: Miðgarðr), or Sigurd.