There are 3 rods all found by talking to fishermans in these areas, dewford town, east of mauville city use surf its the one facing the water to the left, and mossdeep city in a house somewhere i think.
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you just get a rod it doesnt matter what rod it is at the place where there are 2 gym leaders search around and a fisherman will give you one
You can get Horsea by fishing on routes 132, 133, or 134 in Ruby. You may be able to catch it with a Good Rod but a Super Rod would be your best bet.
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.
To get the Fishing rod, you must talk to the fisherman guy in Dewford Town, he will give you one so that you can go fishing.
You get the fishing rod at your home after the elite four.