You don't want the old rod. It only catches Magicarp.
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Yes.
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.
The Old Rod is unobtainable in Pokemon Black and White.
The Old Rod is unobtainable in Pokemon Black and White.
The old rod is south of the city with the Sprout Tower in a Pokemon Center The Good rod is in the city with the Lighthouse in a house with a fisherman who gives it to you (i think)