I'm afraid not, once you migrate Pokemon to diamond and pearl they can never be returned to their original GBA games.
No matter what Pokemon it is and if it is shiny it will stay shiny if you trade it or migrate it, trust me i know i traded my shiny banette to Pokemon xd and its still shiny.
Shaymin was only introduced in the 4th generation games (diamond, pearl, and platinum) and cannot be found or traded to Pokemon emerald.
The description on the Pokemon will either say it came from cinnibar island or route 111 or it will say met in a distant land or met in a trade whichever it says. The region for firered and leafgreen is kanto and ruby/sapphire and emerald is hoenn.
Sadly, you can not. You have to migrate from Saphire or Emerald, or if you have traded it to a Ruby, from there.
Alot
If you own a copy of Pokemon xd gale of darkness and beat it and purified shadow lugia and have traded it to either firered, leafgreen, ruby, sapphire or emerald you can migrate Lugia to diamond then catch it in the pal park to receive it.
Pokemon Emerald can trade with Pokemon versions ruby and sapphire through the use of a trade cable. Emerald can use Wireless adapters to trade with FireRed and LeafGreen aswell. Pokemon from emerald can be permanantly traded to Pokemon games Platinum, Pearl and Diamond by using the GBA slot.. =D I hope this helps =D
There are only a couple of Pokemon that can be traded over to diamond, none of the other Pokemon can be traded over.
You can't.Burmy is a generation 4 pokemon, it did not exist in games before Diamond and Pearl. It cannot be caught in earlier versions such as Emerald, even if you use a cheat device.It cannot be traded to Emerald from the 4th generation games- trading from D/P to Emerald is impossible.Therefore, it is impossible to obtain a Burmy in Pokemon Emerald.
It must be traded to you from emerald or Pokemon colosseum.
Meditite can not be obtained in Pokemon Emerald. It can be traded from Ruby/Sapphire/colosseum/or XD.
Slowpoke cannot be caught in Pokemon Diamond. This Pokemon can be found on route 205 in Pokemon Pearl using the pokemon radar and traded to Diamond.