Arceus can't learn Roost legally, but if you somehow get it to learn it, it will become ??? or Normal type.
>If a Pokémon that has become a pure Flying-type uses Roost, it will become a ???-type Pokémon until the end of the turn.
>If a pure Flying-type Pokémon uses Roost, it will become Normal-type until the end of the turn..