That'd be Arceus.
If you have Arceus, then all you have to do is give it one of the plates that can be collected around Sinnoh, for example, the Splash Plate. Then the Arceus will change its type.
The plates are items that can increase the power of different types of moves. But when you catch arceus, if you put different plates on him, his type and color will change according to the plate.
If held by the Pokemon Arceus the plate will change its type! Normally Arceus is a normal type Pokemon but with the plates you can change it to any of the other 16 types! If given to any other Pokémon, I believe it strengthens moves of the plate's corrosponding type by 20% (E.g. the Splash Plate will power up Water type moves, increasing their damage by 20%)
you can get about 300 diffrent types
ice/snow types
shier piller
Yes Arceas can change types with the plates u find on u'r journy
fighting types and bug types are the most effective types on dark.
Heatran is a fire and steel type pokemon. And Heatran's types do not change in different games. They are the same in Platinum, Pearl, Diamond, HeartGold, and SoulSilver.
The best Pokemon to have in Pokemon diamond is Arceus because he has the best stats in Pokemon diamond. Answer 2 Marowak and Latios are better. Answer 3 Dragonite would have to be the best! It has great stats, is immune to a lot of types and can learn many moves like fire-type moves to deal with super effective ice-types.
Fire types are good against Steel,Ice,Bug,and Grass Pokemon.
no. secret keys are only for changing Rotom's form into a refrigerator, a lawnmower, an oven, types will also change ETC.