I'm pretty sure Arceus never transformed into a Dragon tye. What you may mean is his rage? Where he shoots dragon rage at everybody? Well, that my sir, is one of the Arceus' moves, which according to the anime logic, he had had learnt when he had the Dragon Plate. He is no more Dragon type, at that moment, doesnt mean he can't even use a different type of move, than Normal.
Hope I helped :D