in the lake of rage
you kan have him in shiny
or you catch a magikarp and train him to lv. 20
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It is not possible to get the Red Gyarados in Pokémon Yellow version. You can only get it in Gold, Silver and Crystal version.
Gyarados learns Hyper Beam at level 50.
Magikarp will evolve into Gyarados.
If you are playing the original Gold game: You caught either a Missingno. (I'm pretty sure you didn't) or you caught the Red Gyarados. It's sad, but the Red Gyarados in Gold has a horrible glitch in it that has a chance of doing what you just described. If you are playing Heart Gold: I have no idea what happened. The Gyarados glitch does not occur in Heart Gold.
A golden Margikarp is a shiny margikarp, and as with other shiny Pokemon, there is only a 1/1892 chance of getting a Shiny(golden) magikarp. Breeding a Shiny Gyarados or a Shiny margikarp with another shiny Pokemon THAT CAN BREED WITH GYARADOS/MARGIKARP may have a higher chance of breeding a shiny magikarp. You can also trade the shiny gyarados from Pokemon Gold, Silver, and Crystal version.