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Multitype is hardcoded to only work for Arceus. The wielder changes its type on switching in depending on the Plate held—each Plate corresponds to a different type. The wielder is immune to all item removal effects, so long as it is holding a Plate: Covet, Knock Off, Switcheroo, Thief, and Trick. If the wielder is not currently holding a Plate, it cannot be given one by means of Bestow, Switcheroo, or Trick. This ability can't be copied, replaced, or changed by Mummy, Role Play, Skill Swap, Trace, or similar effects. It cannot be suppressed by Gastro Acid or Worry Seed.
This ability is insane, as it literally makes a Pokemon uncounterable. It makes it even worse that the only user is the Pokemon with the highest Base Stat Total in the game: Arceus. There are hundreds of possibilities opened up thanks to Multitype, so it needs no mention really that it is one of the best abilities in the game.
Found in Dream World