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

Rock Tunnel: Catchable

Mystery Gift (expired): v3.1 (851r3b64a)

You can also receive one in the pokemon center, section "Battle ready non-shiny pokemon."

Shiny Onix
3 shiny Onix are available after accomplishing Shiny Hunter 1. There is no Mystery Gift code--it is given directly from the achievements page to the account. 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.

Onix has a 0.0125% of being shiny in the Rock Tunnel

A Shiny Onix can be purchased through the Pokemon Adoption center for 1 SnD Coin.

Trivia
A giant Onix is an optional boss of Rock Tunnel

Onix doesn't evolve into a steelix in PTD.