lots of things
you can catch a barboach in Pokemon ruby by fishing on the bridge to the west of Fallarbor town wit ha good or super rod
I think you can use a good rod not sure about the old rod super rod is the best
magikarp.....
Super rod.
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.
You can catch Magikarp in any body of water in Pokemon ruby. It is very common and you only need to fish with an old or good rod.
You can use a good rod or super rod to find carvanha on the route southwest from fortree city.
wailmer and sharpedo
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.
In Pokemon Ruby you can catch Wailmer on almost every Route that has lots of water. You will need the Good Rod or the Super Rod to fish one up. The Good Rod can be received from a fisherman on Route 118. The Super Rod is given to you by a fisherman in Mossdeep City.
just get a good rod and catch a tentacool in seawater
There are three types of fishing rods in Pokemon Ruby. The Good Rod is found by talking to a fisherman on Route 118.