Feebas is a Fish Pokémon. It has brown skin with a pattern that resembles a Chocolate Chip Cookie. It also has blue fins on its sides and on the top of its head that looks as if its fin was torn. There is also a fin on the back of this Pokémon. Their characteristics are very similar to a Magikarp.
Feebas' Shiny counterpart has purple skin with a Blueberry Cookie-like pattern as opposed to brown skin and Chocolate Chip Cookie-like pattern as well as the fins being colored grey rather than blue.
Feebas have the special ability Swift Swim. This increases Feebas' Speed when it is raining. Feebas are considered the ugliest and weakest Pokémon in the Hoenn and Sinnoh regions. Unlike Magikarp, Feebas can learn TMs and HMs. Feebas was meant to be the weakest Pokémon.
Feebas evolves into Milotic when its Beauty stat reaches the maximum. In order to do that, the trainer would need to feed it beauty poffins to increase its beauty stat. It will still need to level up. In Pokémon Black and White as well as X and Y, Feebas evolves by being traded holding the item Prism Scale.
Feebas has similarities to Magikarp. Both are known as the Fish Pokémon, both know the same level up moves, both have the same stats, and both evolve into a serpentine Pokémon.
Feebas is the first and only Pokémon so far to change its method of evolution from one generation to another.
In the original R/S/E, Feebas can only be found on 6 random tiles (that change!) on the entirety of Route 119. You would have to fish is those specific tiles in order to find one.
Feebas probably got its name from the word "Feeble" probably because of its low stats, and "bass".
Feebas appears to be based on a bass and on the tale of The Ugly Duckling on whom first is ugly, but then turns out to be a beautiful swan which is much like Feebas evolves into Milotic which is considered the most beautiful Pokémon ever.