Route 23: 34 species in 5 games

Road 23: 34 species across 9 versions

Red and Green (Japan): 12 species
Species	Method	Levels	Chance	Game page
Spearow	Walking in tall grass or a cave	26	15%	All places
Fearow	Walking in tall grass or a cave	38 to 43	25%	All places
Ekans	Walking in tall grass or a cave	26	20%	All places
Arbok	Walking in tall grass or a cave	41	5%	All places
Poliwag	Fishing with a good rod	10	50%	All places
Slowbro	Fishing with a super rod	23	25%	All places
Kingler	Fishing with a super rod	23	25%	All places
Seadra	Fishing with a super rod	23	25%	All places
Goldeen	Fishing with a good rod	10	50%	All places
Seaking	Fishing with a super rod	23	25%	All places
Magikarp	Fishing with an old rod	5	100%	All places
Ditto	Walking in tall grass or a cave	33 to 43	35%	All places

Red and Blue: 12 species
Species	Method	Levels	Chance	Game page
Spearow	Walking in tall grass or a cave	26	15%	All places
Fearow	Walking in tall grass or a cave	38 to 43	25%	All places
Ekans	Walking in tall grass or a cave	26	20%	All places
Arbok	Walking in tall grass or a cave	41	5%	All places
Poliwag	Fishing with a good rod	10	50%	All places
Slowbro	Fishing with a super rod	23	25%	All places
Kingler	Fishing with a super rod	23	25%	All places
Seadra	Fishing with a super rod	23	25%	All places
Goldeen	Fishing with a good rod	10	50%	All places
Seaking	Fishing with a super rod	23	25%	All places
Magikarp	Fishing with an old rod	5	100%	All places
Ditto	Walking in tall grass or a cave	33 to 43	35%	All places

Yellow: 9 species
Species	Method	Levels	Chance	Game page
Fearow	Walking in tall grass or a cave	40 to 45	15%	All places
Nidorina	Walking in tall grass or a cave	41 to 44	30%	All places
Nidorino	Walking in tall grass or a cave	41 to 44	30%	All places
Mankey	Walking in tall grass or a cave	36 to 41	20%	All places
Primeape	Walking in tall grass or a cave	41 to 46	5%	All places
Poliwag	Fishing with a good rod	10	50%	All places
Poliwag	Fishing with a super rod	25 to 30	70%	All places
Poliwhirl	Fishing with a super rod	30 to 40	30%	All places
Goldeen	Fishing with a good rod	10	50%	All places
Magikarp	Fishing with an old rod	5	100%	All places

FireRed: 12 species
Species	Method	Levels	Chance	Game page
Spearow	Walking in tall grass or a cave	32 to 34	15%	All places
Fearow	Walking in tall grass or a cave	40 to 44	25%	All places
Ekans	Walking in tall grass or a cave	32 to 34	20%	All places
Arbok	Walking in tall grass or a cave	44	5%	All places
Psyduck	Fishing with a super rod	15 to 35	5%	All places
Psyduck	Surfing	20 to 40	100%	All places
Mankey	Walking in tall grass or a cave	32 to 34	30%	All places
Primeape	Walking in tall grass or a cave	42	5%	All places
Poliwag	Fishing with a good rod	5 to 15	60%	All places
Poliwag	Fishing with a super rod	15 to 25	40%	All places
Poliwhirl	Fishing with a super rod	20 to 30	40%	All places
Goldeen	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with an old rod	5	100%	All places
Gyarados	Fishing with a super rod	15 to 25	15%	All places

LeafGreen: 12 species
Species	Method	Levels	Chance	Game page
Spearow	Walking in tall grass or a cave	32 to 34	15%	All places
Fearow	Walking in tall grass or a cave	40 to 44	25%	All places
Sandshrew	Walking in tall grass or a cave	32 to 34	20%	All places
Sandslash	Walking in tall grass or a cave	44	5%	All places
Mankey	Walking in tall grass or a cave	32 to 34	30%	All places
Primeape	Walking in tall grass or a cave	42	5%	All places
Poliwag	Fishing with a good rod	5 to 15	60%	All places
Poliwag	Fishing with a super rod	15 to 25	40%	All places
Poliwhirl	Fishing with a super rod	20 to 30	40%	All places
Slowpoke	Fishing with a super rod	15 to 35	5%	All places
Slowpoke	Surfing	20 to 40	100%	All places
Goldeen	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with a good rod	5 to 15	20%	All places
Magikarp	Fishing with an old rod	5	100%	All places
Gyarados	Fishing with a super rod	15 to 25	15%	All places

Let's Go, Pikachu and Eevee: 19 species
Species	Method	Levels	Chance	Game page
Charizard	Rare spawn in the sky with lure or catch combo	3 to 56	100%	All places
Spearow	Walking in the overworld	41 to 46	100%	All places
Spearow	Flying pokemon in the sky	3 to 56	100%	All places
Fearow	Walking in the overworld	41 to 46	100%	All places
Fearow	Flying pokemon in the sky	3 to 56	100%	All places
Nidoran♀	Walking in the overworld	41 to 46	100%	All places
Nidorina	Walking in the overworld	41 to 46	100%	All places
Nidoqueen	Walking in the overworld	41 to 46	100%	All places
Nidoran♂	Walking in the overworld	41 to 46	100%	All places
Nidorino	Walking in the overworld	41 to 46	100%	All places
Nidoking	Walking in the overworld	41 to 46	100%	All places
Poliwag	Surfing in the overworld	41 to 46	100%	All places
Poliwhirl	Surfing in the overworld	41 to 46	100%	All places
Exeggcute	Walking in the overworld	41 to 46	100%	All places
Exeggutor	Walking in the overworld	41 to 46	100%	All places
Chansey	Rare spawn in grass with lure or catch combo	41 to 46	100%	All places
Magikarp	Surfing in the overworld	41 to 46	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