Lairon

The bulgasari, the monster from Korean mythology that feeds on metal and grows the more it swallows, is behind this creature; film versions of the beast added dragon traits that are still present here. The Pokédex entries make the reference pretty obvious. It loves eating iron, feeding on the iron found in rocks and water, and tempers its steel body by drinking nutrient-rich mineral water until it can't take any more.

That explains its way of life. It nests in mountains where iron is mined and near springs, which is exactly where people work, and several entries mention frequent territorial disputes with miners. They settle things by ramming into each other: they smash their bodies together until one gives in, leaving dents and scratches in their armor that never go away. When it wants to show off its strength, it rubs against large rocks to make sparks fly.

It stands 90 centimeters tall and weighs 120 kilograms, a stocky gray quadruped officially classified as the Iron Armor Pokémon. It occupies the middle stage of a three-stage line. It evolves from Aron at level 32 and stays there for only ten levels, until it becomes Aggron at 42.

Its 430 base points are distributed awkwardly. The 140 Defense is the stat that matters, and with 90 Attack it can hit reasonably hard. The problem is on the other side of the sheet: 60 HP and 50 Special Defense. Any moderately powerful special attack cuts straight through it without those 140 points coming into play. Its 40 Speed finishes the job: it almost always moves second.

The Steel and Rock combination has only two weaknesses, one of the shortest lists around, with the catch that both are four times effective: Fighting and Ground. In return, it's immune to Poison and takes only a quarter damage from Normal and Flying. Since Earthquake and Rock Smash are everywhere, that short weakness list is misleading.

Sturdy guarantees it a turn when it's at full health, and that turn is enough for Stealth Rock by TM, Iron Defense at level 58, or a direct hit with Iron Head, learned at 28, and Rock Slide by TM. Rock Head is the other option, cancelling the recoil from Take Down (level 40) and Double-Edge (level 70). Heavy Metal, its Hidden Ability, doubles its weight to 240 kilograms so Heavy Slam hits harder, while also making it more vulnerable to Fighting moves that calculate damage based on the opponent's weight, which already hit it four times as hard.

Since it isn't fully evolved, its place is in formats that allow Eviolite, where that Defense becomes awkward to break physically. In a normal playthrough, it works as a mid-game wall: it switches in against things that attack with their bodies, takes the hit, and retreats as soon as something shows up that attacks from the special side.

It has a single Egg Group, Monster, which pairs it with the lines of heavy four-legged creatures and not much else. The gender split is perfectly even, half and half, so finding a female is no trouble. The egg is the expensive part: 35 cycles, about 9180 steps, making it one of the slowest to hatch.

Its Egg Move list is empty. Everything it knows comes from leveling up or TMs, so breeding here is only useful for natures and IVs; there isn't a single inherited attack that justifies the trip to the Day Care.

Its evolution has no trick or unusual condition. Aron evolves into Lairon when it reaches level 32, and Lairon evolves into Aggron when it reaches 42: no items, no time of day, and no friendship involved. With a Slow growth rate, those levels arrive later than in most lines, and the final stretch drags on.

Its habitat is the mountains, which fits with what the entries say about mines and springs. Its catch rate, 90, puts it in no-man's-land: neither the trivial 255 of a first-route Pokémon nor the demanding 45 of a starter. A little damage and a couple of extra Balls are enough. Wild specimens sometimes hold a Hard Stone, and defeating it awards 2 Defense EVs.