You can't find it in Mt moon.
You can find it in Mt. Moon 1st floor and Mt. Moon B2 floor. They are ultra hard to find though bacause they have a 1.7% appearance ratio. If you find one, try not to kill it or run from it unless you are out of poke balls. There is a very low chance you will find two. You can also buy one with coins at the game corner in Celadon city for 750 coins. -Actually in mt. moon in B2 i found about 15 in 30 minutes, including 3 in a row.
Actually i think your refering to Fire Red or Leaf Green, you can catch onix though the cave to celadon i believe it's Mt. Moon idk i haven't played in a while or Victory Road and i don't think you can catch Marowak but when you battle your Rival in Lavender Town in the Tower of the Dead Pokemon they say they just caught a Cubone, just look for one raise it and evolve it. Hope i helped. no emerald is a different version... you get onix in victory road, same with marowak.
You can find an onix in the oreburgh mine and the iron island in diamond,pearl and platinum.You can get it in the rock tunnel in fire re and leaf green. You dont find onix in ruby,sapphire and emerald.
There are not any.....
a stone onix will evolve into a stone steelixs?
You find it in Mt. Moon
you find onix at cliff and union cave
a stone that you find in mt moon that evloves clafairy to cafable
in the centre part of Mt moon, to get there you have to battle a scientist or fanatic and behind him are the fossils
Mt. Silver, Victory road......most caves
You can find them in mt. moon after you beat the first gym
in mt moon
The best place to find them is Mt Moon, especially at a night. They can also be found on Mt Coronet in Sinnoh as well.
victory road or mt. silver or mt. moon
You can find some in Mt. Moon
Go through Mt. Moon.
You can find an Onix in Dark Cave or in Victory Road in the main game.