The Old Rod is one of three different Fishing Rods you can obtain during your journey of the Kanto Region. Unfortunately, the only Pokemon you will ever be able to obtain by fishing in waters with the Old Rod is the Magikarp. However, even though Magikarp is the weakest Pokemon in all of the Kanto Region, it does Evolve into Gyarados, which just so happens to be one of the strongest Pokemon found within the Kanto Region. In order to obtain the Old Rod, simply talk to the Fisherman in Vermilion City.