Dude, there is a
totally easy way to get a golden Magikarp.
Head to the Lake of Rage, and circle around in the lake three times (no more, no less). After that, defeat seventeen Magikarp in the lake, because 17 is the number of the golden Magikarp.
Use a Repel (because you must not face more than seventeen) and go talk to the old man walking about near the water's edge. He will say something like "I once heard of a cave where golden Magikarp flop and splash around. You need to defeat seventeen regular Magikarp to get it. I saw you defeat the Magikarp seventeen. The cave will open!"
Now you need to Surf up to the north of the lake, and you will hear a thunderbolt. A cave will open. Inside, you have to answer six riddle to progress. The answers are:
1. Pikachu
2. Definitely not
3. I have no heard of such an act, but judging by the description I certainly wouldn't be willing to experiment it with my mother.
4. Potion.
5. A lump of green.
6. Mewtwo
Then you will enter a section of the cave where you encounter golden Magikarp. Be careful though, they all have 126 Attack and know Outrage.
Golden Magikarp only evolve into Gyarados when they feel like it, too. When one evolves, you will see an explanation saying "Oh? Your golden Magikarp has decided to evolve?" Occasionally, this phrase will be followed by "Nah, just kidding", and it won't evolve, but usually it will.
Note - The Gyarados that evolves from the golden Magikarp is a normal blue one, but it suffers from severe acne and comes with a unique move called "Acne Burst", which has a 10% chance of grossing-out the opposing pokemon down to 1HP.
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