You trade it from Diamond, pearl, or platinum. There is no other way. (Without the usage of cheat codes, of course.)
You can't find registeel in Pokemon HeartGold.
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.
unkowns {pokemon}
You cannot find Latios in Pokemon HeartGold unless you went on wi-fi and got the Enigma Stone
yes
Just find them!
You can't
You can't do it.
You can't
trade
There is no such thing.
Find and then catch