Some how you have to get only the rainbow badge, you need a rapidash. GO to bills house and when you go infront of the door, a rainbow will apear, you will ride rapidash up it and will be in rainbow island.
Rainbow island Version-1
Fusion pokemon apear like venustoise, others like pikablue, togepie, some level 300+
Rainbow island Version-2
Game starts over but all the pokemon's stats are higher.
This is the first BS story i ever herd, but i wish it was real, would have been nice to see fusion pokemon