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.
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Pokemon Diamond & Pearl: By surfing - Golduck Using Old Rod - Magikarp Using Good Rod - Magikarp and Goldeen Using Super Rod - Gyarados and Seaking Pokemon Platinum: By Surfing - Golduck and Psyduck Using Old, Good, or Super Rod - Magikarp
It's not what Pokemon the Pokemon surfing is, it's what kind of rod you have. There is a old rod, good rod, and super rod.
Super rod.
Super rod.