Arceus's name may come from the words arch (highest) and deus, which is Latin for god. It may also have a root in the Latin word arcanus, which means secret; sacred mystery. Arceus's name may also come from the word archaic (ancient), which comes from the Greek arkhaios, and is derived from the ancient Greek word for "beginning", arkhē. It could have also come from the Latin word archetypus (original). Alternatively, Arceus's name can be seen as a portmanteau of Zeus, the most powerful of Greek gods and arch (highest). Another theory may be the combination of Zeus and Ra spelled backwards (Zeus being the king of gods and Ra being the solar god and creator, albeit in different pantheons). Those two qualities are shared by the Pokémon. Also, Arceus can also be derived from aureus (golden), possibly referring to its hooves and wheel. Nevertheless, Arceus's name origin and possible theories have led fans to believe that Arceus may be in fact a creator deity in Pokémon form. Arceus's name is pronounced with a soft "c" (or "s" sound) by the announcer in Pokémon Battle Revolution; in keeping with its probable name origin, it is pronounced with a hard "c" (or "k" sound) in television commercials and in Arceus and the Jewel of Life. Its name could also come from arc, as in round/ring, since it's surrounded by one.
-From Bulbapedia