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You have to ask this question first.
What is a pokemon?
Arceus may very well not be a pokemon, or he could be, or simply the classification for what is or isn't a pokemon is very sloppy in the pokemon world.
We have no idea how reliable the taxonomy of pokemon scientists are, considering they've let 3 major types slide by them very easily.
Don't assume pokemon scientists are reliable. They are only working with limited knowledge. They probably don't even consider arceus to be real.