Onix

Description
Onix resembles a giant chain of gray boulders, with a rocky spine on its head. The Pokémon has a magnet in its brain that serves as an internal compass. Onix's body becomes more rounded and smoother as it grows with age. A fully grown Onix measures almost 30 feet.

Location
Pewter Gym: Boss

Mt. Moon 2: Boss

Achievements: Shiny Hunter 1

Mystery Gift: v3.1 (code: 851r3b64)

Shiny
A shiny Onix is only available after accomplishing Shiny Hunter 1. There is no mystery code--it is given directly from the achievments page to the account.

It seems that, if you don't save right away and you have to re-load, it will NOT give you another one to replace the shiny onix that was lost.