Question

Which are the legendaries Pokemon in Platinum?

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Already asked http://pokemondb.net/pokebase/14165/what-legendaries-can-you-catch-in-platinum

Answer 1

These are all the legendaries that are(or were) obtainable in Pokemon Platinum.

First One (whoot whoot)
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Giratina Lv. 47 Distortion World

After battling and catching or defeating Giratina-
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Azelf Lv. 50 Lake Valor

Mesprit Lv. 50 Lake Verity(after talking to it it flees and roams Sinnoh)

Uxie Lv. 50 Lake Acuity

After getting the National Pokedex, talk to Cynthia's grandmother and then get the Adamant and Lustrous Orbs to get these-
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Dialga Lv. 70 (Adamant Orb needed) Spear Pillar

Palkia Lv. 70 (Lustrous Orb needed) Spear Pillar

After getting the National Dex-
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Heatran Lv. 50 Stark Mountain

Cresselia Lv. 50 Fullmoon Island (after talking to it is flees and roams Sinnoh)

Artincuno Lv. 60 Roaming Pokemon

Moltres Lv. 60 Roaming Pokemon

Zapdos Lv. 60 Roaming Pokemon

Regigigas Lv. 1 Snowpoint Temple (must have all three of the regi-trio with you)

Regirock Lv. 30 Special Cave on Route 228 (must have TRU Regigigas in party to obtain)

Regice Lv. 30 Special Cave in Mt. Coronet (must have TRU Regigigas in party to obtain)

Registeel Lv. 30 Special Cave on Iron Island (must have TRU Regigigas in party to obtain)

Event Pokemon (no longer obtainable except through hacking)
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Darkrai Lv. 50 New Moon Island - Membership Pass needed

Shaymin Lv. 30 Flower Paradise - Oak's Letter

Arceus Lv. 80 Spear Pillar - Azure Flute

Aaaand that is all.

Answer 2

the legendaries you can get without events and transfering are:

Giratina
Uxie
Azelf
Mesprit
Dialga(after national dex, you have to go to chyntia s grandmother and she will tell you something about the spear pillar and you have to climb to the summit)
Palkia(same as Dialga)
Heatran(after national dex)
Cresselia(after national dex)
Articuno(prof oak should tell you something, but the bird trio is a roaming legendary like cresselia)
Zapdos(same as articuno)
Moltres(same as articuno and zapdos)

then there are the event only and the ones that you need to transfer from previous games
which are:

Regirock
Regice
Registeel
which then with those 3 you then can get regigigas if you go to the cave where the sandstorm is above the resort area
Darkrai-event only
Shaymin-event only
Arceus-event only

for more information go on this site
http://www.serebii.net/platinum/legends.shtml