In Pokémon Sun and Moon, Feebas evolves into Milotic through a specific method. Here's how you can evolve Feebas in these games:
Catch a Feebas: Feebas can be found in only one specific location in Sun and Moon. You need to go to Route 1 and fish for Feebas in the various fishing spots along the route. Note that Feebas is quite rare, so it may take some time and patience to encounter one.
Increase Feebas's Beauty: Feebas evolves based on its Beauty stat. In order to evolve it, you need to increase its Beauty as much as possible. You can do this by feeding it special Pokéblocks or using Beauty-enhancing items like Blue Scarves or Beauty Drinks. These items can be obtained at the Battle Royal Dome in Royal Avenue.
Find a Prism Scale: Once you've raised Feebas's Beauty to a high level, you need to obtain a Prism Scale. This item is necessary for Feebas to evolve into Milotic.
Trade Feebas while holding the Prism Scale: With a Feebas that has high Beauty and a Prism Scale in your possession, initiate a trade with another player or a secondary game. During the trade, make sure that Feebas is holding the Prism Scale. After the trade is completed, Feebas will evolve into Milotic on the other player's game.
It's important to note that Feebas's Beauty can be checked by showing it to a specific character in the game. In Pokémon Sun and Moon, this character is in the Hau'oli City Pokémon Center.
Remember, evolving Feebas into Milotic requires raising its Beauty and trading it while holding a Prism Scale. This method applies specifically to Pokémon Sun and Moon, and the process may be different in other Pokémon games.