Multitype changes the form and type of a Pokémon with this Ability depending on the Plate attached.
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⚠️Gastro Acid, Worry Seed, and Simple Beam fail if the target has the Ability Multitype.
⚠️Role Play, Skill Swap, Entrainment, and Doodle fail if either Pokémon has Multitype.
⚠️Multitype cannot be copied by Trace, Power of Alchemy, or Receiver. ⚠️Multitype cannot be replaced by Mummy or Lingering Aroma, suppressed by Core Enforcer or Neutralizing Gas, or traded by Wandering Spirit.
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🟣Gen⁴
⚠️The item-removing effects of Thief, Covet, Trick, Switcheroo, and Knock Off will not work on a Pokémon with Multitype, regardless of what item it is holding.
🎨When a Pokémon transforms into Arceus, it becomes the same form and type as the target Arceus.
🎨If the transformed Pokémon is holding a Plate that is different from the target's held Plate, it then immediately changes to the form and type corresponding to that Plate.
🎨If it is not holding a Plate, it immediately changes to Normal Arceus (unless it transformed into Normal Arceus).
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🔵Gen⁵⁻⁶
🎨If a Pokémon transforms into Arceus, the Pokémon will now be in the same form as the target Arceus, regardless of its own held item.
🎨Multitype no longer has an effect on items; instead, item-manipulating effects cannot affect Arceus if they would cause it to hold a Plate or take a Plate it already holds.
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🔴Gen⁷
🎨Multitype will also change Arceus's type and form if it holds a type-specific Z-Crystal.
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🟠Gen⁹
🎨If the user Terastallizes, the effects of Multitype will be nullified.
~Bulbapedia