The support page for Pokemon revealed that the players will need to walk Feebas as a Pokemon buddy if they want to evolve it into Milotic. The Buddy Evolutions are uncommon in Pokemon Go, but it is natural because Feebas requires a high “Beauty” condition before it can evolve.
You have to walk Feebas as your buddy for 20km and have 100 candy to evolve it to Milotic. It takes 5km to get a candy with Feebas as your buddy.
Feebas is the third one who needs a special treatment before evolving.
Before this, players needed to walk Eeevee for 10km as a buddy, if they wanted to evolve it into Espeon or Umbreon. This is a complete mimic of the “Friendship” metric, which was introduced in Pokemon Silver and Gold.
Many thought that Feebas would need an evolutionary item to evolve in the games. The later generations of the games required that Feebas has to be traded with a Prism Scale.
Other players thought that Pokemon Go will add the Prism Scale as evolutionary items which could go together with Deepsea Scale and Deepsea Tooth (coming soon in Pokemon Go).
For now, Feebas is the only one with walking distance evolution requirement in the Game_Master.