Cloyster

It swallows seawater and expels it under pressure through a tube at the back of its shell. That is how it moves, and it also fires the spikes around its body through the same tube: propulsion and weapon come from the same mechanism. The rest is armor. The Pokédex entries insist that it cannot be scratched, that it can withstand the impact of a bomb, and that it only opens when attacked. Nothing is known about the body inside; the official text itself admits that its shape remains a mystery.

The spikes are not just decoration, either. When the shell is wounded, it grows a new, sharp spike as it heals, so a battered specimen ends up better armed than a peaceful one, and those living in strong ocean currents grow the longest spikes of all. Someone realized their potential long ago: spears made from those spikes have been unearthed, used by ancient hunter peoples.

It is 1.5 metres tall and weighs 132.5 kilograms, almost all of it shell, and the Pokédex classifies it as the Bivalve Pokémon. It closes a two-stage evolutionary line. Shellder is the little clam that keeps sticking out its tongue; Cloyster is what happens when that clam touches a Water Stone and decides never to open again.

Its 525 base stat total is deliberately lopsided: 180 Defense, the highest figure in the entire first generation, against 45 Special Defense and 50 HP. On the physical side, barely anything can move it; on the special side, almost anything takes it down. The rest—95 Attack, 85 Special Attack and 70 Speed—does not stand out on its own.

What makes it usable is Skill Link, which forces multi-hit moves to always hit five times. Icicle Spear, with 25 power per hit, becomes a powerful and reliable Ice-type attack; Rock Blast does the same for Rock. On top of that, it has Shell Smash, which doubles Attack, Special Attack and Speed at the cost of dropping both defenses by two stages. With those 180 Defense points, it can set up in front of a physical attacker and survive, and White Herb cancels the drop immediately.

In Scarlet and Violet, Smogon places it in RU with the same setup: Skill Link, Shell Smash, Icicle Spear, Drill Run for Steel-types and Tera Blast with an Electric or Ghost Tera Type. In Monotype, it has a place on Water teams, where its Defense lets it set up in front of physical opponents, although it competes there with options that can hit hard without needing a setup turn.

The drawback is its typing. Water and Ice resist two things—Water and Ice—and are weak to Grass, Fighting, Rock and Electric. After Shell Smash, any priority move or moderately powerful special attack takes it down. Its weakness to Rock also punishes it when switching in, which is why the RU set considers Heavy-Duty Boots as an alternative to White Herb.

It belongs to a single Egg Group, Water 3, the one for marine invertebrates, which cuts down the list of compatible partners considerably; outside that group, only Ditto remains. The gender ratio is straightforward, 50% female, so getting a specimen of the desired sex adds no extra work.

The 20 incubation cycles equal roughly 5355 steps, a high figure for a Pokémon that is neither Legendary nor pseudo-Legendary. It has no Egg Moves: everything it knows comes from levelling up or TMs, and its level-up movepool is almost entirely concentrated at Lv.1—including Hydro Pump, Ice Beam, Shell Smash, Icicle Crash, Iron Defense, Spikes and Toxic Spikes. Icicle Spear appears at Lv.0, meaning it is only available through the Move Reminder. The only move it learns later is Razor Shell, at level 5.

Its evolution is a simple one: use a Water Stone on Shellder, with no minimum level, no friendship requirement and no time-of-day condition. Since Cloyster barely learns anything by levelling up, when to use the stone depends not on it, but on what Shellder still has left to learn.

Its habitat is the sea. Its catch rate, 60, is closer to the 45 of difficult-to-catch Pokémon than to the 255 of those that fall to a Poké Ball on the first try. It grows at a slow rate, yields 2 Defense EVs and may be holding a Pearl or Big Pearl in the wild.