When you have different GBA Cartridges in your GBA Slot (FireRed, LeafGreen, Ruby, Sapphire or Emerald) you can catch wild Pokémon in different locations, relevant to the GBA game you have inserted. You can get Pokémon like Solrock (Ruby), Lotad (Sapphire), Weedle (FireRed), Caterpie (LeafGreen) and Gligar (Emerald) this way.