What legendary Pokemon can you get on Pokemon Fire Red?

Answer 1

The legendary birds: Moltres (Mt. Ember) Zapdos (Power Plant) and Articuno (Seafoam Islands). You can also find Mewtwo in the Unknown Dungeon which you gain access to after beating the elite four and giving the ruby and the sapphire to Celio. After beating the elite four for the second time, one of the legendary beasts (more commonly known as the legendary dogs) Raikou, Suicune or Entei will be released and running all over the world. Which one you get is random and I don't think there's a way you can "choose". Hope that helps!

EDIT: You can determine what legendary dog you can get. If you start with Charmander you will find Suicune. If you start with Squirtle you will find Raikou. If you start with Bulbasaur you will find Entei. I would save your master ball for them for they are much harder to catch than the other 4 legendaries.