The Legendary Pokémon in this game are of small amount, so here is a probably incomplete list of the Legendary Pokémon you can catch in Fire Red/Leaf Green and where to get them

Articuno

Articuno is found on Seafoam Island as always. It is Level 50.

It is an Ice/Flying type so bring a Fire/Electric of roughly an equal level

Zapdos

Zapdos is found in the Power Plant as always. It is Level 50.

It is an Electric/Flying type so bring a Rock/Ice Pkmn of roughly an equal level

Moltres

Moltres is different this time and is found on Rainbow Island 1. It is Level 50.

It is a Fire/Flying type so bring a Water Pkmn of roughly an equal level

Mewtwo

Mewtwo is found in the Unknown Dungeon as always. It is Level 70. You need to beat the Elite 4, have the National Dex and fixed the Network Machine on Island 1

It is a Psychic type so bring a Dark or Bug Pkmn (such as Tyranitar or Heracross) of roughly an equal level

Raikou

After Beating the Elite 4 you could see Raikou running around the grasses of Kanto. It appears if you picked Squirtle as your Starter. It is Level 50.

It is an Electric type so bring a Ground Pokémon of roughly an equal level

Entei

After Beating the Elite 4 you could see Entei running around the grasses of Kanto. It appears if you picked Bulbasaur as your Starter. It is Level 50.

It is an Fire type so bring a Water/Rock Pokémon of roughly an equal level

Suicune

After Beating the Elite 4 you could see Suicune running around the grasses of Kanto. It appears if you picked Charmander as your Starter. It is Level 50.

It is a Water type so bring a Grass/Electric Pokémon of roughly an equal level

Ho-oh

Situated on Rainbow Island 8, Ho-oh is on the Left and on the top. It is Level 70.

It is a Fire/Flying type so bring a Water of roughly an equal level

Lugia

Situated on Rainbow Island 8, Lugia is on the Right and on the top. It is Level 70.

It is a Psychic/Flying type so bring a Dark/Rock of roughly an equal level

Deoxys

Situated on Rainbow Island 9, Deoxys appears after you complete a Puzzle. It is Level 30.

It is a Psychic type so bring a Dark of roughly an equal level

 
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