Legendary Pokemon
There are very few Pokemon in Red, Blue, and Yellow whose rarity is so extreme that legends are created for them. You can find these Pokemon hidden throughout Kanto, waiting at specific spots. These Pokemon are incredibly difficult to capture. Always save your game before interacting with these Pokemon. Try reducing their health to the lowest amount as possible, inflict the Sleep status condition, and start throwing all the Ultra Balls you have.
Articuno
Articuno is one of the three Legendary birds in Pokemon Red, Blue, and Yellow. This Pokemon can be found waiting at the bottom of Seafoam Islands. Before you can reach it, you’ll need to push a few boulders down several holes to block off the rapid river to Articuno.
Zapdos
Zapdos is one of the three Legendary birds in Pokemon Red, Blue, and Yellow. This Pokemon can be found waiting near the exit of the Power Plant, which is located downstream on the east side of Route 10.
Moltres
Moltres is one of the three Legendary birds in Pokemon Red, Blue, and Yellow. This Pokemon can be found waiting near the exit of Victory Road.
Mewtwo
Mewtwo is the strongest Pokemon in Red, Blue, and Yellow. He can be found waiting at the bottom of the Unknown Dungeon, also known as Cerulean Cave, which is west of Cerulean City. A man will be blocking the entrance to the Unknown Dungeon until you become the Pokemon League Champion.