Regigigas is a Legendary Pokemon. You can get Regigigas by finding the three Regi's Regirock Registeel and Regice you need to put all three in your party then talk to the guy with the crown in Indigo Plateau. If you want to get a shiny Regigigas then buy the Shinys gamepass to get a good chance to get one or you can get one by luck. You can not get more then one Regigigas from the guy if u trade yours away the only way to get another is by Trading.