Arceus

Arceus is a white, equine Pokémon with an androgynous appearance. It resembles a qilin or centaur with a gray, vertically striated underside. The striated pattern also appears on the underside of its mane, tail, and face. Its four pointed feet are tipped with gold hooves. Its long mane juts away from its head, and its face is gray, with green eyes, red pupils, and a green circular pattern below its eyes. Arceus also has a streak of gold coloration on its head, which points upward. Its neck is fairly long, with two pairs of extrusions to the sides, and a flap-like feature on the neck's underside that is colored white like much of the body. Arceus also has a golden cross-like wheel attached to its body by its round abdomen. The wheel also has four green jewels attached to it. The pattern of striated gray of Arceus's underbelly continues past its waist. Its limbs have gray undersides and extrusions at the tops of the legs. Arceus's tail is similar to its mane in terms of shape and coloration. It is capable of flight.

According to Sinnoh mythology, a single Pokémon Egg emerged from a vortex of complete chaos and nothingness predating the Pokémon universe. Arceus hatched from this egg and shaped the universe using its 1,000 arms, including the Sinnoh (then known as Hisui) and Ransei regions. It also created the eggs from which the Pokémon of myth, Dialga, Palkia, and Giratina, and the lake guardians, Uxie, Azelf, and Mesprit, hatched from, and can recreate these eggs if it wants to. It is the oldest Pokémon in existence; as such, it is referred to as "Original One" in folklore. Arceus banished Giratina to the Distortion World due to its violent nature, and gave the lake guardians the power to control and calm either Dialga or Palkia should they become enraged. Like the Pokémon of myth, Arceus also resides in another dimension, a realm beyond time and space itself. There, it appears as a silhouette of golden light.

Arceus was also shown to be capable of speech, stating that "Arceus" is simply the name humans call it. The ancient Sinnoh people worshipped it, referring to it as "almighty Sinnoh," "great Sinnoh," and "might of Sinnoh," with the term "Sinnoh" later on becoming the name of the region. The Diamond Clan and Pearl Clan also worshipped it, though they mistook Dialga and Palkia respectively for Arceus. Both Palkia and Dialga's Origin Formes have body shapes similar to that of Arceus; this resemblance is believed to be a strategy in order to gain Arceus' powers. Professor Laventon describes Arceus as "the heavenly fount from which pours the light that shines across Hisui" and the protector of all Pokémon. In ancient myths, it is said that the Origin Ore, an ore of mysterious power, contains the power of Arceus itself.

As shown in Arceus and the Jewel of Life (M12), Arceus was known to care for and safeguard the planet in ancient times, protecting the world from cataclysms such as meteors, and it would show gratitude and compassion to those who treated it with kindness. At the same time, Arceus does not suffer fools gladly; it will attack in a violent rampage if betrayed or deceived and holds long grudges if betrayed by those it considered friends. In modern times, Arceus continues to watch over the world and wishes for people and Pokémon to work together in the face of adversity. Even when it is needed, it only gives hints on how to solve problems instead of directly interfering. Arceus can also give its blessing to other Pokémon through bolts of light, making them bigger and more powerful, as seen with the ancient hero's 10 Pokémon companions. This blessing is passed down through family lines, as seen with the descendants of the hero's Pokémon: the noble Pokémon and ride Pokémon worshiped by the people of Hisui.

In Arceus and the Jewel of Life, Arceus is portrayed as having powers such as bringing back things that it once destroyed, making certain things vanish into thin air, halting time, and overturning the very laws of nature. In said movie, using its Earth, Splash, Meadow, Zap, and Draco Plates, it created the Jewel of Life, a powerful artifact capable of turning a desolate land into a rich and fertile one. In Hoopa and the Clash of Ages (M18), it was shown to be able to stop the destruction caused by a space-time void. As shown in the Pokémon Journeys: The Series episode Showdown at the Gates of Warp! (JN090), Arceus has the power to not only calm Dialga and Palkia, but also destroy the Red Chain controlling them. Arceus also has the power to split in two, creating a piece of itself to exist outside of its realm. In Pokémon Legends: Arceus, Arceus transformed a normal smartphone into an Arc Phone, a device that allows humans to communicate with it through text messages. Arceus also has the power to appear in dreams and create a variety of Pokémon to serve as challenges, as seen in Eternal Battle Reverie. In Pokémon Masters EX, through the sync stone ultimate, Arceus can give its blessing to a Pokémon Trainer it deems worthy, transforming their outfits into costumes known as arc suits. As a result, their sync pair is blessed as well, granting them a divine move over their normal sync move.

Forms

Arceus has eighteen different forms, one for each type.

Through its Multitype Ability, Arceus can change its type at will by holding one of its Plates or a type-specific Z-Crystal. This causes its eyes, underbelly, face, hooves, snout, gems, and cross-like wheel to change color to match its current type. In Arceus and the Jewel of Life, it is also said to feed off the energy of its Plates.

In Alola, the Aether Foundation gathered Arceus's data from myths and legends told in Canalave City's library to create an artificial Pokémon who was initially named Type: Full. Equipped with the RKS System, an Ability that allows it to simulate Arceus's Multitype through memories, it was developed during the Beast killer project to protect Alola from the Ultra Beasts. However, the project was deemed a failure and was later abandoned, and Type: Full's Ability was sealed with a control helmet, which caused it to devolve and be renamed Type: Null. Gladion eventually discovered it and renamed it Silvally, with their newfound bond allowing it to reawaken its dormant powers.

(Source: Bulbapedia)