Dome Fossil

A fossilized Pokémon shell.

Revive this at Pewter City Museum to get Kabuto. Evolves into Kabutops, which is great and can run Swift Swim for rain teams!

The Mt. Moon fossil choice comes down to physical attacker (Dome/Kabutops) vs special attacker (Helix/Omastar) essentially. You can pick what you need and Wonder Trade for the other.