How do you get the golden magikarp?

Answer 1

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.