You can find him in Mt. Coronent if you have Pokemon Pearl or Diamond version but if you have Ruby,Emerald,or Sapphire then you can find him in the waters of Route 119 though it will be very hard to find it Good Luck!
the rarest Pokemon in Ruby Sapphire and Emerald is Feebas
Only in ruby, sapphire and emerald at route 119
Since Feebas is a Water Pokemon,you can only find it in lakes,rivers etc.
On the Route southwest from Fortree City.
in lake verity toss the rod in and a feebas will appear
you can find a feebas in route 118 and 119 but its hard to find
the rarest Pokemon in Ruby Sapphire and Emerald is Feebas
the only way to evolve it is to have the feebas at a high BEAUTY status for Pokemon contests. and the best way to do this is to have a feebas with a MODEST nature.
trade it from ruby or sapphire
You must evolve Feebas.
blue pokeblocks to max beauty
Modest is really great!
you can get feebas on route 119 very rare
Feebas cannot be caught in Pokemon LeafGreen. You would have to trade one from Ruby, Sapphire, or Emerald.
Feebas is found on the patch of water in the route to the LEFT of Fortree, in 6 Random squares, which change when the Dewford trendy phrase is changed. You will need to fish for Feebas.
Not including the legendaries and special pokemon ( ones that you can only catch once ) feebas is the hardest and rarest pokemon to catch
Depending on where you use it you can find various pokemon like Carvanha, Corphish, Sharpedo, Feebas, Staryu.