To get the shiny onix all you have to do is (and I highly recomend it ) get a haunter to at least lvl. 35/36 and get it to learn nightshade.
After you have done this equip either a master ball or a lot of ultra balls and go to victory road and save it. Just walk around for a while.
If your fortunate enough you'll be attacked by a silver onix.
Use haunter and use nightshade twice at least until its health is low and try to catch it.
It really works I have it and I'm not lying I promise.
P.S is there any other shiny pokemon besides red gyarados and silver onix? If there is then post here on SC !
Added by: pokemonvs.neopets Jan 27th 2005, ID#3257