Inserting a Generation III game will let you encounter some different Pokemon, but only in specific locations.
Ruby
Haunter (Old Chateau)
Gengar (Old Chateau)
Seedot (Routes 203, 204, 210, Eterna Forest)
Nuzleaf (Route 210, 229)
Mawile (Iron Island
Zangoose (Route 208, 210)
Solrock (Lake Verity, Lake Acuity, Lake Valor, Mt. Coronet, Sendoff Spring, Turnback Cave)
Sapphire
Haunter (Old Chateau)
Gengar (Old Chateau)
Lotad (Route 203, 204, 205, 212)
Lombre (Route 212, 229)
Sableye (Iron Island)
Seviper (Route 208, 210)
Lunatone (Lake Verity, Lake Acuity, Lake Valor, Mt. Coronet, Sendoff Spring, Turnback Cave)
Emerald
Haunter (Old Chateau)
Gengar (Old Chateau)
Teddiursa (Route 211, Acuity Lakefront [DP], Lake Acuity [Pt]
Ursaring (Route 216, 217, Acuity Lakefront [Pt], Lake Acuity [DP]
Pineco (Eterna Forest, Route 203, 204, 210, 229)
Gligar (Stark Mountain, Route 206, 207, 214, 215, 227)
Shuckle (Route 224)
Staravia (Lake Verity)
Bibarel (Lake Verity)
FireRed
Caterpie (Route 204, Eterna Forest [Pt])
Metapod (Eterna Forest)
Ekans (Route 212)
Arbok (Great Marsh)
Growlithe (Route 201, 202)
Haunter (Old Chateau)
Gengar (Old Chateau)
Elekid (Route 205, Valley Windworks)
Staravia (Lake Verity)
Bibarel (Lake Verity)
LeafGreen
Weedle (Route 204, Eterna Forest [Pt])
Kakuna (Eterna Forest)
Sandshrew (Wayward Cave)
Sandslash (Route 228)
Vulpix (Route 209, 214)
Haunter (Old Chateau)
Gengar (Old Chateau)
Magby (Route 227, Stark Mountain)
Staravia (Lake Verity)
Bibarel (Lake Verity)
Dual-slot mode (Bulbapedia)