Even though Pokemon FireRed takes place in the Kanto Region, the Pokemon from the first three Pokemon Generations are coded within the game since it was released during the 3rd Generation. This means that even though most cannot be found naturally in the Wild within Pokemon FireRed, all the Legendary Pokemon from the 2nd Generation and 3rd Generation can be traded for and obtained. Believe it or not, one piece of the Johto Legendary Beast Trio (Raikou, Entei, Suicune) can actually be found without having been a Pokemon Event or by using Cheats Codes.
After completing the Main Story and having defeated Team Rocket in the Sevii Islands, the Legendary Pokemon Suicun will begin roaming the Kanto Region if you chose Charmander as your Starter Pokemon. If found, it will be at Level 50 and will know the moves Bubblebeam, Rain Dance, Gust, and Aurora Beam. Unfortunately, it will attempt to leave on the first turn of the battle. Because of this, it would be wise to have a Pokemon that knows the move Block at the lead of your Pokemon Party. This move will prevent Suicune from fleeing the battle when encountered.
Since Suicune is so very hard to find alone, it would be especially wise to wait to use your Master Ball on it. Unlike Entei and Raikou, however, Suicune can't be prevented from fleeing in battle by using the move known as Block. This means that one could attempt to paralyze it and use Ultra Balls to try to capture it, allowing one to save their Master Ball for the likes of Mewtwo (or even Lugia or Ho-Oh).
Click Here To Learn How To Catch Raikou, The Next Piece Of The Legendary Beast Trio!