Arceus seems to have as many names as it does powers, but one of them has to be its true name. As the deity tells players in the intro to Pokémon Legends: Arceus, the name "Arceus" is simply what humans have chosen to call it. According to Arceus itself, its true name has no equivalent in human language. The mysterious Legendary Pokémon also has multiple mystical stories that have filled the Pokémon world and fanbase. All of these contribute to the mystery surrounding this pivotal creature, not only in the game but in the diegetic world of Pokémon.
Arceus believes it hatched from an egg, possibly a reference to the world egg myth, the belief that either the universe itself or some primordial creator hatched from an egg. Arceus is said to have created not just the Sinnoh region but the Pokémon universe as a whole - a bit of Pokémon lore retconned by Legends: Arceus - as Arceus itself was born before the universe. From that point it created the Lake Guardians and Creation Trio, and those six helped shape the world and its inhabitants.SCREENRANT VIDEO OF THE DAY
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A common theory after Pokémon: Legends' release is that Arceus' true name is Sinnoh. Minus the fact this would not fit the criteria of its name having no human language equivalent, this can be debunked for another reason. In Pokémon Legends: Arceus, the Pearl Clan worships the almighty Sinnoh who created space, while the Diamond Clan worships the almighty Sinnoh who created time. This can easily be interpreted to mean that they worship Palkia and Dialga respectively. So while people eventually call Pokémon Legends: Arceus' Hisui region Sinnoh in honor of the eponymous deity, that was never really its name to begin with.
Arceus Appears To Be A Combination Of Religious Figures
Other possible names for Arceus take a bit of work digging into the various religions of the real world, as some theorize Arceus is meant to represent a conjoining of prominent gods from various religions. Such unofficial information has been collected on Bulbapedia and the Pokémon Wiki. Some of Arceus' Pokédex entries say it is believed to have crafted the universe with its 1,000 arms, a reference to the bodhisattva Avalokiteśvara. The arc around Arceus' back could be inspired by the bhāvacakra, a Buddhist symbol used to represent cyclical life and reincarnation. Said arc could also be pictured as representing a halo, which has long been used in various religious iconography to symbolize the divine. Arceus' design combining three Legendary Pokémon hints at it being a sort of divine amalgamation. The etymology of Arceus can also be linked to different gods. Arceus' name holds a resemblance to important deities such as Zeus, ruler of Mount Olympus in Greek mythology, Uranus, the Greco-Roman god of the skies, and even Jesus, Christianity's central figure. Additionally, Arceus' name on its own can be deciphered to read as alpha or original divine being.
It's unlikely Arceus' true name is just the Pokémon version of an already existing god. Perhaps Arceus is something new entirely, some form of a combination of religions and myth - a knot that, in a sense, ties all these different beliefs into one being for a universe that is played by trainers of different backgrounds. If none of these is the true answer then only two options remain: Arceus' true name will never be known (unless Nintendo reveals it in the next Pokémon Legends game), or its true name is left up to the player. Every trainer will have their own name for Arceus, and if such is the case, everything and nothing is Arceus' true name.