Route 9: 31 species in 8 games

Road 9: 31 species across 14 versions

Red and Green (Japan): 3 species, all by walking in tall grass or a cave
Species	Levels	Chance	Game page
Rattata	14 to 17	40%	All places
Spearow	13 to 17	35%	All places
Ekans	11 to 17	25%	All places

Red and Blue: 3 species, all by walking in tall grass or a cave
Species	Levels	Chance	Game page
Rattata	14 to 17	40%	All places
Spearow	13 to 17	35%	All places
Ekans	11 to 17	25%	All places

Yellow: 8 species, all by walking in tall grass or a cave
Species	Levels	Chance	Game page
Rattata	18	15%	All places
Raticate	20	4%	All places
Spearow	17	10%	All places
Fearow	19	1%	All places
Nidoran♀	16 to 18	30%	All places
Nidorina	18	5%	All places
Nidoran♂	16 to 18	30%	All places
Nidorino	18	5%	All places

Gold: 9 species
Species	Method	Levels	Chance	Game page
Rattata	Walking in tall grass or a cave	13 to 15	up to 40%	All places
Raticate	Walking in tall grass or a cave	15	up to 25%	All places
Spearow	Walking in tall grass or a cave	13	up to 20%	All places
Fearow	Walking in tall grass or a cave	15	up to 5%	All places
Mankey	Walking in tall grass or a cave	13	up to 30%	All places
Primeape	Walking in tall grass or a cave	15	up to 5%	All places
Goldeen	Fishing with a good rod	20	65%	All places
Goldeen	Fishing with an old rod	10	15%	All places
Goldeen	Fishing with a super rod	40	70%	All places
Goldeen	Surfing	10 to 19	90%	All places
Seaking	Fishing with a super rod	40	10%	All places
Seaking	Surfing	15 to 19	10%	All places
Magikarp	Fishing with a good rod	20	35%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Magikarp	Fishing with a super rod	40	20%	All places

Silver: 7 species
Species	Method	Levels	Chance	Game page
Rattata	Walking in tall grass or a cave	13 to 15	up to 70%	All places
Raticate	Walking in tall grass or a cave	15	up to 30%	All places
Spearow	Walking in tall grass or a cave	13	up to 20%	All places
Fearow	Walking in tall grass or a cave	15	up to 5%	All places
Goldeen	Fishing with a good rod	20	65%	All places
Goldeen	Fishing with an old rod	10	15%	All places
Goldeen	Fishing with a super rod	40	70%	All places
Goldeen	Surfing	10 to 19	90%	All places
Seaking	Fishing with a super rod	40	10%	All places
Seaking	Surfing	15 to 19	10%	All places
Magikarp	Fishing with a good rod	20	35%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Magikarp	Fishing with a super rod	40	20%	All places

Crystal: 11 species
Species	Method	Levels	Chance	Game page
Rattata	Walking in tall grass or a cave	15	up to 30%	All places
Raticate	Walking in tall grass or a cave	15 to 18	up to 25%	All places
Spearow	Walking in tall grass or a cave	15	up to 30%	All places
Fearow	Walking in tall grass or a cave	15	up to 15%	All places
Zubat	Walking in tall grass or a cave	15	5%	All places
Venonat	Walking in tall grass or a cave	15	30%	All places
Venomoth	Walking in tall grass or a cave	15	10%	All places
Marowak	Walking in tall grass or a cave	18	up to 5%	All places
Goldeen	Fishing with a good rod	20	65%	All places
Goldeen	Fishing with an old rod	10	15%	All places
Goldeen	Fishing with a super rod	40	70%	All places
Goldeen	Surfing	10 to 19	90%	All places
Seaking	Fishing with a super rod	40	10%	All places
Seaking	Surfing	15 to 19	10%	All places
Magikarp	Fishing with a good rod	20	35%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Magikarp	Fishing with a super rod	40	20%	All places

FireRed and LeafGreen: 3 species, all by walking in tall grass or a cave
Species	Levels	Chance	Game page
Rattata	14 to 17	40%	All places
Spearow	13 to 17	35%	All places
Ekans	11 to 17	25%	All places

HeartGold: 14 species
Species	Method	Levels	Chance	Game page
Rattata	Walking in tall grass or a cave	15	up to 20%	All places
Raticate	Walking in tall grass or a cave	13 to 15	up to 20%	All places
Spearow	Walking in tall grass or a cave	13	up to 20%	All places
Fearow	Walking in tall grass or a cave	15	up to 5%	All places
Mankey	Walking in tall grass or a cave	13	up to 20%	All places
Primeape	Walking in tall grass or a cave	15	5%	All places
Goldeen	Fishing with a good rod	20	60%	All places
Goldeen	Fishing with an old rod	10	15%	All places
Goldeen	Fishing with a super rod	40	70%	All places
Goldeen	Surfing	10 to 15	90%	All places
Seaking	Fishing with a super rod	40	10%	All places
Seaking	Surfing	15	10%	All places
Magikarp	Fishing with a good rod	20	40%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Magikarp	Fishing with a super rod	40	20%	All places
Linoone	Walking in tall grass or a cave	13	20%	All places
Whismur	Walking in tall grass or a cave	13 to 15	20%	All places
Sableye	Walking in tall grass or a cave	13 to 15	40%	All places
Bidoof	Walking in tall grass or a cave	13	20%	All places
Buizel	Walking in tall grass or a cave	13 to 15	20%	All places

SoulSilver: 12 species
Species	Method	Levels	Chance	Game page
Rattata	Walking in tall grass or a cave	13 to 15	up to 20%	All places
Raticate	Walking in tall grass or a cave	13 to 15	up to 20%	All places
Spearow	Walking in tall grass or a cave	13 to 15	up to 30%	All places
Fearow	Walking in tall grass or a cave	15	up to 5%	All places
Goldeen	Fishing with a good rod	20	60%	All places
Goldeen	Fishing with an old rod	10	15%	All places
Goldeen	Fishing with a super rod	40	70%	All places
Goldeen	Surfing	10 to 15	90%	All places
Seaking	Fishing with a super rod	40	10%	All places
Seaking	Surfing	15	10%	All places
Magikarp	Fishing with a good rod	20	40%	All places
Magikarp	Fishing with an old rod	10	85%	All places
Magikarp	Fishing with a super rod	40	20%	All places
Linoone	Walking in tall grass or a cave	13	20%	All places
Whismur	Walking in tall grass or a cave	13 to 15	20%	All places
Mawile	Walking in tall grass or a cave	13 to 15	40%	All places
Bidoof	Walking in tall grass or a cave	13	20%	All places
Buizel	Walking in tall grass or a cave	13 to 15	20%	All places

Let's Go, Pikachu and Eevee: 14 species
Species	Method	Levels	Chance	Game page
Charizard	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Rattata	Walking in the overworld	17 to 22	100%	All places
Raticate	Walking in the overworld	17 to 22	100%	All places
Spearow	Walking in the overworld	17 to 22	100%	All places
Spearow	Flying pokemon in the sky	3 to 56	100%	All places
Fearow	Walking in the overworld	17 to 22	100%	All places
Fearow	Flying pokemon in the sky	3 to 56	100%	All places
Nidoran♀	Walking in the overworld	17 to 22	100%	All places
Nidorina	Walking in the overworld	17 to 22	100%	All places
Nidoran♂	Walking in the overworld	17 to 22	100%	All places
Nidorino	Walking in the overworld	17 to 22	100%	All places
Chansey	Rare spawn in grass with lure or catch combo	17 to 22	100%	All places
Articuno	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Zapdos	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Moltres	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Dragonite	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places