Feebas is a Water-type Pokemon that can be rarely fished from the pond located at Fortulose Manor, and has the abilities Swift Swim, Oblivious, or the hidden ability Adaptability.
Feebas evolves into Milotic when traded with a Prism Scale, which can be randomly acquired from the meals at New Sushi Stick located in the Anthian City shopping district. Be mindful as each helping of sushi costs $5,000.
Head over to route 9, and go to the door that leads to the mansion. Once there, head to the little pond and fish. It will take a few tries, but you'll get a feebas soon.
[How to quickly evolve it]
Head to the sushi place in Anthian City. Eat sushi from the man (but it is 5,000 each) and you'll get a fortune cookie. It will vary, so I suggest you save before buying it, then leave when you're low in money. THEN come back again and retry until you get Prism Scale. Once you have that, click 'give' and then give it to feebas. Go to the Trade Resort. (You will need a partner you can FULLY TRUST!) Trade your Feebas that is equipped with the scale with your partner, then wait a few seconds.