The legend states that Ho-oh revived three Pokemon that lived in its tower when it was burned. The three Pokemon were also given the forms of the three beasts, but they were never always like that (seriously. Suicune could have quenched the flames and Entei couldn't have been burnt).
I'm guessing Raikou was an Ampharos, Suicune maybe a Slowking and Entei maybe being an Arcanine (lets say he was crushed by the building then).
They could also have been a Jolteon, Vaporeon and Flareon though. But Eevee's not even catchable in the wild. I bet the Mansion owner guy just buys a bunch of them and releases them in his garden...