What legendary Pokemon can you get in Fire Red?

Answer 1

Legendary Pokemon you can catch normally in FireRed and LeafGreen:

#144 Articuno @ Seafoam Islands

#145 Zapdos @ The Powerplant

#146 Moltres @ Mt. Ember

#150 Mewtwo @ Cerulean Cave

#243 Raikou (If you chose Squirtle as your starter Pokemon) Roams Kanto

#244 Entei (If you chose Bulbasaur as your starter Pokemon) Roams Kanto

#245 Suicune (If you chose Charmander as your starter Pokemon) Roams Kanto

Raikou, Entei, and Suicune require you to have beaten the Elite Four.

This is the same for Mewtwo.