You can catch:-
Kanto Based Legendaries
Articuno Seafoam Islands
Zapdos Power Plant
Moltres Mount Silver
Mewtwo Cerulean Cave
Mew Special Event
Johto Based Legendaries
Raikou Roaming Johto
Entei Roaming Johto
Suicune Roaming Johto and Kanto
Ho-Oh Tin Tower
Lugia Whirl Islands
Celebi Ilex Forest (Event)
Heonn Based Legendaries
Kyogre Embedded Tower (Blue Orb, HeartGold)
Groudon Embedded Tower (Red Orb, SoulSilver)
Rayquaza Embedded Tower (Jade Orb)
Deoxys Birth Island
Latios Roaming Johto and Kanto (SoulSilver) Event
Latias Roaming Johto and Kanto (HeartGold) Event
Sinnoh Based Legendaries
Dialga Sinjoh Ruins (Arceus Event)
Palkia Sinjoh Ruins (Arceus Event)
Giratina Sinjoh Ruins (Arceus Event
Arceus Sinjoh Ruins (Arceus Event)
*This answer has been approved and written by Wikia Lord.
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The cool legendaries you can get in Soulsilver/heartgold are the following: Lugia, Ho-oh, Raikou, Entei, Suicune, Articuno, Moltres, Zapdos, Mewtwo, Latias, Latios (Latios in Soulsilver & Latias in Heartgold), Kyogre, Groudon, Rayquaza (Kyogre in Heartgold, Groudon in Soulsilver & Rayquaza if you have both).
there is a mew in Pokemon soulsilver but you need to catch all leadndery Pokemon including event Pokemon
Almost all pokemon with evolutions can be bred. Legendaries can not though.
the legendaries can be found when you defeated all gyms
all except unow and all legendaries