Pokémon Black Version and Pokémon White Version - Species Restrictions
If a Pokémon is an Arceus and hasn’t transformed, effects (other than Pickup) can’t cause it to gain a Plate, and if it’s actually holding a Plate, effects can’t cause it to remove that item.
If a Pokémon is a Giratina and hasn’t transformed, effects (other than Pickup) can’t cause it to gain a Griseous Orb, and if it’s actually holding a Griseous Orb, effects can’t cause it to remove that item.
If a Pokémon is a Genesect and hasn’t transformed, effects (other than Pickup) can’t cause it to gain a Drive, and if it’s actually holding a Drive, effects can’t cause it to remove that item.
If Pickup causes Arceus, Giratina, or Genesect to gain a Plate, a Griseous Orb, or a Drive, respectively, it doesn’t change its form.