You can find Raikou in any patch of grass after you have beaten the elite 4 a second time after obtaining the national dex (requires you to have caught 60 Pokemon).
The legendary cat that can appear depends on the Pokémon you choose in the beginning of the game. If you chose Bulbasaur then you can get Entie (fire), if you chose Charmander then you can catch suicine (water) and if you chose Squirtle you can catch Raikou (electric) near Fuchsia City in the strips of grass by the ledge near the entrance.
Raikou randomly appears around the world map after you have beaten the Elite Four, and if you chose Squirtle as your starter Pokemon.
First,take a bulbasaur as a starter and after you beat the league, just roam around in the grass and eventually you wiil find him. He flees just like Latios and Latias.
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Trade from FireRed or LeafGreen.
You cannot catch Raikou in Pokemon Diamond and Pearl. To get Raikou in Pokemon Diamond and Pearl, you must trade for one from Pokemon HeartGold and SoulSilver where it can be caught or transferring from Pokemon FireRed and LeafGreen.
You will find Raikou in the wild once you have defeated the Elite Four. Note that the legendaries are as follows: Suicune: Charmander Raikou: Squirtle Entei: ulasasour
It depends on who your starter Pokemon is. If you start of with bulbasaur, entei will appear, if you start with squirtle, Raikou will appear, and if you start of will charmander, suicune will appear. Those 3 Pokemon could anywhere in the game, so you will have to find that Pokemon yourself.
You cant find it in Firered only leafgreen.