That Pokemon is Shaymin,which is only avadible through cheating or hacking,or a Nintendo Event.
After you do the second to last gym it becomes available
if you meen the national dex then you need to find every Pokemon in the sinnoh pokedex
in the grass in the last city the 8th gym leader
manaphee
in the Pokemon center up the escelator the last lady you find
A Pokemon adventure on Pokemon Diamond Verison can last forever if you wanted it to.
Arceus
No you can not trade from GBA games to Pokemon Diamond/Pearl but instead you can migrate.If you mean the original Pokemon Red (as in the second version to ever be released) then no.If you mean Pokemon FireRed, the last answerer was accurate in saying you can't trade, but migrate.
the same pokemon they had last time
Garatina...
Palkia is the last Pokemon in pearl and in diamond you find him in a book that an old lady shows you and in pearl you find at spear pillar on mount coronet. (I really hope this helps you on your wild adventure)
Pokemon diamond and pearl chocha culo teta bicho