Once the player completes the main storyline and obtains the National Pokedex, Entei, Raikou and Suicune become available to capture as roaming legendaries. During an encounter with one of them, if the player uses Mean Look and one of the legendary dogs uses Roar, it will end the battle as usual. However, the Pokemon will have disappeared from the map upon checking the Pokedex to locate the Pokemon, making it impossible to further encounter them in that save file.
Contributed by allymender