You cannot CATCH Celebi on Pokemon HeartGold or SoulSilver, but there is an wifi event a GameStop that gives you a Celebi in mystery gift. You can also use a Action Replay™, but I suggest you do the event.
You can't catch Rotom in heartgold.
you cant catch him in heartgold you can only get him in soulsilver
In HeartGold you can catch Kyogre.
You can only catch Vulpix in SoulSilver, not in HeartGold. You can catch it on route 36 and route 37.
In pokemon Heartgold
you cant catch it it is rare
yes you can catch every Pokemon ever made
You can catch Lombre in Pokemon HeartGold in the Safari Zone at a waterside area once you receive the National Pokedex.
You can catch Ekans in Pokémon HeartGold in the Safari Zone in the Marshlands area.
there is no Pokemon called croagoak