I found this kind of neat. See, you either put ruby, saphire, emerald, fire red, or leaf green pokemon games in the GBA slot while playing Pokemon D/P and these pokemon can be found:
Ruby in GBA Slot:
Seedot- Eterna Forest, Route 203, Route 204.
Nuzleaf- Route 229.
Mawile- Iron Mountain.
Zangoose- Route 208.
Solrock- All lakes, Mt.Coronet, Spring Path, Turnback Cave.
Saphire in GBA Slot:
Lotad- Route 203 and 204
Lombre- Route 229
Sableye- Iron Mountain
Seviper- Route 209
Lunatone- Same as Solrock
Emerald in GBA Slot:
Pineco- Eterna Forest, Route 203, 204, and 229.
Gligar- Stark Mountain, Route 206, 207, 214, and 215
Shuckle- Route 224
Teddiursa- Route 211 and Lake Acuity
Ursaring- Route 216 and 217, Lake Acuity
FireRed in GBA Slot:
Caterpie- Route 204 (South)
Metapod- Eterna Forest
Ekans-Route 212 (South)
Arbok- Safari Game
Growlithe- Route 201 and 202
Elekid- Vally Windworks
LeafGreen in GBA Slot:
Weedle- Route 204 (South)
Kakuna- Eterna Forest
Sandshrew- Wayward Cave
Sandslash- Route 228
Vulpix- Route 209
Magby- Stark Mountain and Route 227
Ummm...I hope this helped a bit...