Magikarp is the only Pokemon you can catch with the old rod, right? Or are there others (I know in previous gens there were sometimes)?
More importantly, has anyone heard anything about the method to catch Feebas? I saw a couple posts about it already but I don't think there was an answer yet. The Hoenn dex says Route 119 again but nothing about its rarity or if it's the same as the original games with the Dewford trending saying controlling the spaces it's at.
Magikarp probably has around a 90% encounter rate or so with the Old Rod, but it is possible to catch other Pokémon--like Tentacool, Goldeen, and others, depending on where you fish--with the Old Rod, as well. The only exception appears to be the waters inside Sootopolis, which houses exclusively Magikarp for every rod except Super, which also provides a small chance of finding Gyarados.
Feebas doesn't have the same mechanic that it did in the originals, it's just a dreadfully low encounter rate with any type of fishing rod. You might have the highest chance with an Old Rod, but only due to process of elimination (meaning, you can only hook Magikarp and Feebas with the Old Rod, but the Good/Super Rods add Carvanha to the encounter pool, lowering the encounter rate of Feebas when using those rods.)
@JSLatios: Lati@s is available after the 5th gym. When you get to Rte. 18, Steven shows up as it approaches the two of you. It communicates to Steven that it's asking for your help at Southern Island, so it flies the two of you there where Team Magma/Aqua is harrassing it over its Mega Stone. You squish the grunts, and it willingly joins your party--no battle, no struggle to capture, not even a roaming hunt. It comes holding its Mega Stone, and Steven gives you the Mega Bracelet right after.