Once you ask the Copycat in Saffron City to get her Clefairy Doll back Talk to the man inside The Pokemon Fan Club.Then he will give you the Clefairy Doll go outside.Then Steven Stone will talk to you he will say that Latias(HeartGold)or Latios(SoulSilver).Finally Latias or Latios will roam Johto or Kanto.But sorry,you can't get Latios without Cheating,Hacking or Trading. :(
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I don't think you can get Latios in Heartgold legitimately. I'm pretty sure you can only get Latios in Heartgold by using an action replay code, such as the Pokemon Modifier Code. You could also trade it over from Soulsilver, I think.
In the pewter city museum. to get his attantion you must bring walking latias ( heartgold) or latios ( soulsilver ) with a soul dew from a mystery gift. then he will take you to saffron city. he will talk to you for a while. then he will go back to pewter then show you latios ( heartgold ) or latias ( soulsilver ) outside of the museum. then after you defeat or catch one of eon Pokemon that's the last of him.
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).
In the new Pokemon HeartGold game, you can catch new Pokemon such as Beldum, Latios/Latias and you can choose between Treeko, Mudkip, or Torchic. (Can all be obtained through Pokemon League Champion Steven from the Hoenn Region.)
You need to get latias ( can only catch it on Pokemon heartgold) and latios (can only get it on Pokemon soulsilver) and speak to steven who is in the silph co. in saffron city.You must get the Enigma Stone from the WiFi event, which turns out to be the Soul Dew.