Does Giratina-O need to hold Griseous Orb in Hackmons?

Question

And do the Arceus forms need to hold the Plates?

Comments

Up vote 'cause I was wondering the same thing.

Answer 1

I'm answering this because the other answer is partly wrong.

In the case of Giratina-O, it does not need to hold Griseous Orb to change its form to Giratina-O.
The other answer was posted in 2013 when regular Hackmons were not implemented, and Classic Hackmons was the only playable Hackmons format on Showdown.
Classic Hackmons was notorious for not exactly replicating in-game possible hacks. The last years of Classic Hackmons were smooth and fundamentally replicated, though.

Arceus needs its Plate and signature ability in Multitype to be able to change its type.
The ability legalities make it not possible to get any Arceus except for Arceus-Normal without the Plates and the ability. Even having only a Plate wouldn't help; that'd just change the type of Arceus' Judgment.
Only Multitype without any Plate is Arceus-Normal. In BDSP, though, Arceus can have all of its types without the need of holding a Plate or having Multitype as its ability.

Answer 2

I just tested it on Showdown.
I selected Giratina-O and gave it Leftovers. It came out as regular form Giratina.

I selected Arceus-Bug and gave it Leftovers. It came out as Normal type.