What do you do after catching Mewtwo on leafgreen?

After beating the Elite 4 for the second time if you haven't already, one of the legendary beasts (Suicune, Entei, Raikou) will be released and be running around the world. Suicune if you started with Charmander, Entei if you started with Bulbasaur or Raikou if you started with Squirtle.