Gold and Silver legendaries

Lugia
You can catch Lugia in the Whirl islands in Silver after you get the Silver Wing. For Gold you must get the Silver Wing from a man in Pewter City.

Ho-oh
You can catch Ho-oh at the top of Bell Tower in Gold after you get the Rainbow Wing. For Silver you must get the Rainbow Wing from a man in Pewter City.

Legendary Dogs
You can catch them after you awaken them in Ecruteak City. They will be moving all across Johto. You can see their location by checking their area in your Pokedex.