Helix Fossil

A fossilized Pokémon shell. Can be revived into Omanyte at the Pewter City Museum.

Revive this to get Omanyte, which evolves into Omastar. Rock/Water with good Special Attack - and Shell Smash makes it scary if you can set up.

You pick between this and Dome Fossil in Mt. Moon. Personally I lean Helix most runs since Omastar's Shell Smash sweeping potential is harder to pass up than Kabutops' physical attacking and could never forget Twitch Plays Pokémon.