Dialga
You'll have to have received it from a special event.
mesprit,uxie,azelf,dialga,rotom only in Pokemon diamond you get cresselia and darkrai from a Nintendo event dratini,dragonair,dragonite is not a legendary giratina you can only get it in platinum
his name is actually Arceus, and he is the most legendary of legendary Pokemon in diamond. he is only obtainable through a Nintendo event with the azure flute
the legendary Pokemon you can catch in pkm d/p is dialga (diamond), palkia (pearl), cresselia, regigigas, heatran, arceus (event pkm), darkrai (event pkm), shaymin (event pkm) , azelf, uxie, mesprit. and giratina
trade with Pokemon Platinum
Celebi is a legendary Psychic and Grass type Pokemon. It can only be obtained by trading or through a Nintendo event.
Without mystery gift event: Rotom, Heatran, Azelf, Mesprit, Uxie, Cresselia, Giratina, Regigas and Dialga WIth event: Darkrai, Shaymin, Manaphy, Phione, and Arceus! (arceus is the best and hardest one!)
you have to catch it and then you you have to trade from sapphire or ruby or emerald to diamond or pearl
Action Replay.
You can get it from an event or trade.
You can't.
You can get Giratina, Dialga, Uxie, Azelf, Mespirit, Heatran, Cresselia, and Regigigass. through Pokemon ranger you can get Manphy. through past events you can get Darkrai, Arceus, and Shaymin. You can get a shiny pichu through an event, he may not be legendary but its still worth noting.