The in-game legendaries are Ho-oh, Luigia, Mewtwo, Entei, Raikou, Suicune, Moltres, Articuno, Zapdos, Latias, Dialga, Giratina, and Palkia (Dialga, Palkia, and Giratina can only be received if you have 12th Movie Arceus, but you can only choose 2 of the 3). The legendaries that you can receive by Mystery Gift are Mew, Latios, Jirachi, and Celebi ( Celebi is at GameStop from February 21, 2011 until March 6, 2011).
Well basically...
Ho-oh after getting the rainbow wing and the clear bell
Entei after encountering in ecruteak city
Raikou after encountering in ecruteak city
Lugia after beating the first 8 gyms in whirl pool islands
Suicune in cerulean city after finding the suicune guy in kanto
Mewtwo in cerulean cave after beating all 16 gyms
Zapdos next to power plant after completing the game
Arcticuno in seafoam islands after completing the game
Moltres in sliver mountain sorry bout the spelling
Ho-oh Level 45 Bell Tower
Lugia Level 70 Whirl Islands
Entei Level 40 Johto Roamer
Raikou Level 40 Johto Roamer
Suicune Level 40 Route 25
Articuno Level 50 Whirl Islands
Zapdos Level 50 Route 10
Moltres Level 50 Mt Silver
Mewtwo Level 70 Cerulean Cave
Kyogre Level 50 Embedded Tower
Latias Level 35 Kanto Roamer
Special legendaries
Dialga/Palkia/Giratina Level 1 Sinjoh Ruins. Show the man at the Ruins of Alph an Arceus.
Rayquaza Level 50 Embedded Tower. Show Professor Oak a Groudon and Kyogre caught at the Embedded Tower.
Latios Level 35 Pewter City. Get Enigma Stone Mystery Gift and show to Pewter Museum.
All other legendaries are Mystery Gift only.
yes at supercheats.com/pokemon heartgold action replay codes
The PokeWalker allows you to walk with a select Pokemon from your copy of HeartGold and if you walk enough, you can capture extremely rare Pokemon (though not legendary).
Yes, you can trade Legendary Pokémon as well as any other Pokémon from Pokémon Platinum over to Pokémon HeartGold, you just need the regional PokéDex in both trades before you can trade.
Aura Beam is in Pokemon HeartGold.
That is impossible in Pokemon HeartGold and SoulSilver.
there are many legendary Pokemon in Pokemon heartgold, but the best is HO-OH
I put a link to a guide all about legendary pokemon in HeartGold in the related links.
In HeartGold you can catch Kyogre.
Articuno
mew
I don't think you can refight any legendary Pokemon except the wandering dogs.
migrate and trade
Kyogre is only available for Pokemon: Sapphire on the GBA. Ho-Oh is the legendary pokemon available to you in HeartGold.
No legendary can lay an egg except for manaphy which gives an egg to phione and you can not get manaphy or phione in heartgold
yes at supercheats.com/pokemon heartgold action replay codes
Ho-ohLugiaCelebiJirachiLatiasLatiosSuicuneRaikouEnteiZapdosMoltresArticunoMewtwoKyogre-HeartgoldGroudon-SoulsilverRayquazaPalkiaDialgaGiratina
Moltres is the only legendary Pokemon in Mt. Silver for HeartGold / SoulSilver only.