Does anyone know why Cloyster is an Ice-type? Seriously, read it's Pokedex entries. Nowhere in the Pokedex does it say something like "It's body can reach temperatures of -200° F" or other generic Ice-type info, nor does it even mention anything along the lines of "It lives in frigid northern waters on the rocky seafloor." Nothing of the sort. This is what it does say, however:
"For protection, it uses its harder-than-diamonds shell. It also shoots spikes from the shell."
-Yellow Pokedex
"Its shell is so hard, it can even withstand a bomb. No one has ever seen what is inside its shell."
-Stadium/FR Pokedex
"Even a missile can't break the spikes it uses to stab opponents. They're even harder than its shell."
-Crystal Pokedex
What's harder than diamond? Nothing you say? Well, this
is Pokemon logic after all, so I would infer that Cloyster's shell is as hard as, if not harder than,
steel. Yes, it gains the Steel-typing. It makes more sense flavorwise in my opinion based on what we know from the Pokedex, and its a great defensive typing as well. It only has two weaknesses, Fighting and Ground. However, unfortunately for it, these two types are also some of the most common in the metagame. Luckily, the Pokedex holds one more tidbit of information that helps Cloyster deal with this...
Cloyster's shell can withstand bombs and missiles. Bulletproof prevents bomb- and bullet-based moves. It's a perfect fit, really. Mainly Cloyster values Bulletproof... (skip paragraphs) Cloyster also lacks reliable recovery. This makes it vulnerable to status (particularly burn)... (continued)