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Barbaroach
go fishing in the great marsh
there is a guy under cycling road or maybe it is a wild pokemon
go fishing in falador town with a good rod of super rod or surf.
The only way you can see all the Pokemon on diamond is by going into the GTS and global trade different Pokemon.
you have to catch all of the pokemon in diamond or SEE all of the pokemon
A Pokemon
The Pokemon you see do not go in your pokedex.
No, you cannot see every Pokémon in the National Dex in Pokémon Diamond however you can see every Pokémon in the regional Sinnoh Dex in Pokémon Diamond.
for diamond and pearl there are 150 Pokemon u need to see . in plat nium u neeed to see 210 Pokemon.
You cannot see it in the wild in Pokemon Diamond; you have to transfer it from FireRed/LeafGreen.
You see it in the Pokemon league The champion has it