Use the Old Rod in any body of water press A when it says you have a bite and you will hook magikarp.
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magikarp.....
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.
Its called a shiny it occurs 1/8146 (i think) out of every Pokemon encounter
You catch Magikarp on route 12.
catch a magikarp and evolve it