Long ago, There was a sturdy tower in the northwest corner of the city of Ecruteak. And as a sturdy tower it stood. Until one terrible night, when lightning struck. As the tower burst into flames, people began to realize that they made a horrible mistake. In the haze of fear during evacuation, three Pokémon were left within. But just then, the Rainbow bird Ho-Oh soared, summoned the rains, and put out the fire.
Ho-Oh then proceeded to bring the three lost Pokémon back. They became Raikou, Entei, and Suicune. Raikou, the lightning that struck the tower. Entei, the flame that consumed the tower. And Suicune, the rain that doused the tower.