It's in the foggy lake in Mt. Coronet. There are only four water blocks where Feebas appears, and they change daily. Feebas can be encountered by finding one of these spots and fishing. It's fairly hard to find because of this, and even if you're in the right spot, it's not guaranteed to bite. Once you hook it, though, it's fairly easy to catch.
mt corenet
super rod
No
the foggy bit in Mt.Cornet
yes
Mt. Coronet; north of celestic town
Old Rod.
Feebas are tricky to catch you can only find them in certain floors in Mt. cornet.
You can catch Feebas in Mt. Coronet on B2F on the north side. You can also find Feebas in atrainers party on route 222.
theres a underground lake deep in mt.cornet
u have to catch a feebas in mt. coronet its in four fishing spots
Fish in Mt. Cornet, the part that takes you to Snowpoint City