The complete Pokemon RBY pokedex, part 9

Pokémon Name: Cloyster
Type: Water/Ice
Classification: Bivalve Pokemon
Pokédex Number: 091
Ability: Shell Armor and Skill Link
Dream World ability: Overcoat
Useful Attacks: Spike Cannon
Location Found:
D/P/P/HG/SS: Evolve from Shellder
B/W: Route 13, Undella Town, Undella Bay (all via Fishing, bubble spots only)

The Skill Link ability lets multi-hit moves hit more often, so keeping the attack Clamp around is a sound strategy. Shellder has quite a few moves that you'll want to keep an eye on, especially since Cloyster only learns Spike Cannon at level 40. Don't evolve until you're ready.

Evolution: From Shellder via Water Stone