Legendary Pokemon in hgss

Ho-oh

You can obtain Ho-oh by receiving the rainbow wing, after you have received the rainbow wing you can then go to the Tin Tower to battle Ho-oh, Ho-oh will be lvl 45, and lvl 70 in SoulSilver.

Lugia

You can obtain Lugia by going to the Whirl islands, Lugia will be lvl 45, and lvl 70 in HeartGold.

Entei, Raikou, and Suicune

Both Entei and Raikou can be obtained by chasing them through the Johto region, both lvl 40.

You can obtain Suicune in specific places.