From Bulbapedia, the community-driven Pokémon encyclopedia.
550 years before the storyline of Generation II took place there were the Tin Tower and the Brass Tower. Ruled by Lugia and Ho-Oh One day lightning struck the Brass Tower and it was on fire for three days. Then there was a sudden downpour that put the fire out. The tower was burned to the gorund, Lugia fled to the whirl islands and three creatures inside of the tower died. One creature representing the lightning the struck the tower known as Raikou. Another that represented the fire that burned it known as Entei. One representing the rain that put the fire out known as Suicune. Ho-Oh resurected them and they set off to find trainers with pure hearts.