You can catch feebas everywhere and occasionally barboach i am unsure where to catch magicarp but there is a way
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Using the Old Rod in Pokemon Ruby (or any Pokemon game for that matter) will get you a Magikarp as that is the only Pokemon an Old Rod can catch.
magikarp.....
Super rod.
Use the Old Rod in any body of water press A when it says you have a bite and you will hook magikarp.
I am looking to join Wailmer to my team.But, no you can't catch it with an old rod