Catching old Pokemon?

[Bolt]

I heard that if you have one of the GBA games in your DS while playing Diamond/Pearl, some older Pokemon will be avilable in the wild. I put Fire Red in cause I want Growlithe, but do I need the National Dex first? I probably do, but I don't know!

 

[bullados]

I think you need the National Dex, then you need to get to a specific area in the game. Some sort of Safari Zone-type place? Idk...

 

[Dek]

For the FireRed, LeafGreen, and Emerald, you need the National Dex since they are actually obtainable in those games.

Otherwise, you just need at least seven Pokemon in your game.

The area in D/P you need to go to is Pal Park (assuming they didn't change the names of the areas)