From Bulbapedia, the community-driven Pokémon encyclopedia.
Multitype (Japanese: マルチタイプ Multitype) is an ability introduced in Generation IV. Only one Pokémon has this ability.
Multitype changes the type of the Pokémon with this ability depending on the Plate attached. This ability cannot be transferred with Skill Swap or copied with Trace. The item-removing effects of Thief, Covet, and Knock Off will not work on a Pokémon with Multitype if it is holding a plate. If any Pokémon other than Arceus has this ability, it does not work.
When a Ditto Transforms into Arceus, Ditto becomes Arceus's Normal form. If it is holding a plate, Ditto will Transform into Arceus's current form and then will change type and appearance to the type corresponding to its plate.
Outside of battle
Multitype has no effect outside of battle.
Pokémon with Multitype
No Pokémon have Multitype as one of their two abilities.