That means if you have a GBA game in your GBA slot then you can get them. If you don't then they won't be there.
Ruby: Seedot, nuzleaf, zangoose, solrock and mawhile.
Saphire: Lotad, lombre, sableye, lunatone, seviper.
Emerald: Pineco, gligar, shuckle, tedyursa, ursaring.
Fire red: caterpie, metapod, ekans, arbok, elekid, growlith.
Lear green: weedle, metapod, sandshrew, sandslash, vulpix, magby.
Any game: Haunter, Gengar.
If you want to know where to find them look on serebii, or ask a seperate question.
