The legendary dogs, Raikou, Entei and Suicune, begin to appear randomly in wild grass once the player has beaten the Elite Four and obtained access to the Sevii Island Ferry. The player can only catch one of the three in any given playthrough, and it depends on the starter Pokémon.
If the player chooses Squirtle as the starter Pokémon, Raikou appears in the game. If the player chooses Bulbasaur as the starter, Entei appears; for Charmander, it is Suicune. All of the legendary dogs are found randomly in wild grass, and their location changes any time the player enters a building or moves into a new area. The best way to find them is to travel up and down some of the longer multiple-area routes in the game.