The first legendary Pokémon you find is Dialga by interacting with it at Spear Pillar.
After that, you can catch the trio Mesprit, Uxie and Azelf. You can find Mesprit in about any tall grass of Sinnoh after meeting it at Lake Verity. (Use the Map Poketech App to see where it is located.)
Uxie can be found at Lake Acuity and Azelf is at Lake Valor.
Giratina is in Turnback Cave.
Cresselia is at Full Moon Island, but it will quickly flee and roam Sinnoh.
Heatran is at the innermost room at Stark Mountain.
Arceus can be found by using the Azure Flute at Spear Pillar. This item never came out as an event, so a cheating device will be required.
Darkrai can be interacted with at Newmoon Island after obtaining the Membership Card (It was an event item, but a cheating device will be required to obtain this item today.) and Shaymin is in Flowery Paradise after obtaining Oak's Letter by using a cheating device, for the item was an event and the event has expired.
Regigigas can be found in Snowpoint Temple after bring the Regi trio.
Here's a little bonus when it comes to once-only Pokemon.
Although, Rotom is not a legendary, it can still be found in the Old Chateau north of Eterna Forest. It will have the same theme as Darkrai, Regigigas, and Heatran. You must enter the top middle room with the TV and your time has to be 8 p.m or later.
cheats
you go to the spear pillar.
it would probably be shaymin because he's low level
Pokemon Diamond was released a tiny bit earlier as it is put first in bulbapedia lists and the creators needed time to change the lengendary from dialga to palkia.
You will need to catch the three lake lengendary pokemon. (Mesprit, Uxie, Azelf).
yes sort of
No.
the lengendary birds are in caves ex. moltres is at mt. ember and the ground lengendary Pokemon pop out of the grass once in the game. it depends on what your starter Pokemon is to what lengendary ground Pokemon you'll get.
If you want to transfer a Pokemon to Pokemon Diamond and Pearl, you will need Nintendo Wi-Fi. You have to download the mission(s) and transfer to your DS Card.
no, but it is awsome!!!
no they cant
Yes. You can. However, you must have traded it from another game. I have about three Rayquazas on my Pokemon Diamond because I have traded them there from all the Sapphire games I have restarted.