Arceus is a Mythical Pokemon.
Do note that early on, there wasn't a huge separation of legendries vs mythicals. You'll see the words intertwined in early gens. The distinction in English wasn't made until Gen 5, so there's been times prior in which Arceus has been referred to as legendary, but he's still always been mythical. Source
Arceus (Japanese: アルセウス Arceus) is a Normal-type Mythical Pokémon introduced in Generation IV.
Pokémon Legends: Arceus
The official English website of Pokémon Legends: Arceus used to refer to Arceus as a Legendary Pokémon, which was later corrected to Mythical Pokémon on September 28, 2021. It was always referred to as a Mythical Pokémon on the Japanese website.
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