To catch a Pokemon with a rod, you have to use the rod, then wait until a "!" appears above your character's head. Once that happens, press "a" as fast as you can before it gets away. If you land the Pokemon, you get to battle it.
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tentacool and magikarp ... As you can also catch goldeen on emerald
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.
on rount 119 note use super rod
Route 128 or Evergrande City (Super Rod)
Go to meteor falls and use a super rod.