Cloyster, a Bivalve Pokémon. Its shell is incredibly strong and resistant to a bomb. Its shell only opens when it attacks.
– Ash’s Pokedex
Cloyster looks kinda mean or evil, I think it’s the grin. It sort of has the same expression as Gengar. Shellder’s shell is horizontally split, but Cloyster’s shell is vertically split, although in the original sprites of Red/Blue, it was horizontally split.
I’m not really sure why Cloyster is part Ice. Perhaps the clam it’s based on lives in cold water. Like the Seel/Dewgong family, Shellder is pure water type, but gains the Ice subtype when it evolves into Cloyster.
I’ve never had a Cloyster before. While its Defense is very good (at 180 base Defense, the highest of Water and Ice types), its other stats are pretty average.
However, with Gen V’s introduction of the move Shell Smash (which raises Speed, Attack and Sp Attack by 2 stages, but lowers Defense and Sp Defense by one stage each), Cloyster has become more competitive. Its ability Skill Link, makes multi-hit moves hit max number of times, so Icicle Spear and Rock Blast would always hit 5 times. And Gen V has improved power and accuracy of those two moves respectively, making them a lot more useful. Icicle Spear with Skill Link Cloyster has a base power of 125, the highest of any Ice type attack (other than Sheer Cold I suppose). Combine that with Shell Smash, Cloyster can turn into a serious sweeper.