Hoopa actually is a legend, and he was introduced with the rings since he was made. His story line was he started out in his regular form, but he began to get bored. Eventually, He started battling other Pokemon and beat them all. He was awarded prizes every time he won, and he began to feel like a god. At this point, he stayed in his unbound form all the time, and he began to be taken over by his dark side. One horrible day, he summoned Every legend there was and battled them in a raging fight. He conquered them all. He grew out of control and almost destroyed the village, but a hero uncapped the prison bottle key and drew in hoopa's evil form. Ever since, that hero raised hoopa to become good, and he sealed away hoopas evil powers with the bottle in a cave deep in the Hoenn region. The purpose of hoopa's rings are they can lead to ANYWHERE in the world, the catch is that hoopa himself can't travel through his rings until he makes peace with his evil side, and finally controls it. They can even lead to other dimensions, and that is how he brought all the legends over to him.