Onix makes its debut as a Pokémon that comes out of a Poké Ball in Super Smash Bros.. Onix uses Rock Throw as its attack. Onix jumps to the top of the screen and release boulders from its body. The rocks will start falling down on the stage. Any character that makes contact with the rocks will take damage. Onix's attack does not harm the summoner.
A life-sized balloon of Onix (in actuality the 3D model of Onix used in Pokémon Stadium for Nintendo 64) is one of the many floating and bending "platforms" that comprise the Poké Floats stage. It is the second platform balloon to appear, right after Squirtle, and all matches on this stage have Onix making its appearance as a temporary flat platform similar to Final Destination.