To evolve Feebas into Milotic, you need to let your Feebas hold a Prism Scale and then trade it to another game. And there are actually two Prism Scales in both X and Y. The first one is hidden near the waterfalls in Couriway Town (use your Dowsing Machine). The second is in Lumiose City in the building next to the Hotel Richissime. On the fourth floor, a child will give it to you. EDIT: 'd...