Posted Mar-20-2017 2:19 PM
(This is just speculative lore of the Primal forms for Dialga, Palkia, and Giratina, based off the forms I created for them.)
When Arceus created Dialga, Palkia, and Giratina, he granted them Primal forms so that they could create the universe. Giratina, however, used its Primal power violently, and so was banished and had its powers suppressed. Once Dialga and Palkia completed their labours of creating time and space, they reverted to their normal forms as they retreated to their dimensions.
As the Era of Creation drew to a close, Arceus created the Adamant, Lustrous, and Griseous Orbs in case the power of Primal Dialga, Palkia, and Giratina would be needed in the future.