Steelix

Males have two pairs of projections on their lower jaws, while females have only one. It's the only visible difference between the sexes in a creature that is otherwise a 9.2-metre, 400-kilogram chain of metal blocks topped with a head that bites through stone. The Pokédex puts it bluntly: it chews its way through rock, and its eyes can see in the darkness beneath the ground.

The rest is explained by the depth at which it lives. It descends deeper than Onix, the Pokémon it evolves from, and Pokédex entries mention records of one kilometre underground. The pressure and heat of that habitat have made its body harder than any metal. The texts disagree on how this happens: one entry points to the iron in the earth it eats, building up over the years until it transforms the Pokémon from within; another claims that an Onix only needs to live past the age of one hundred for its body to acquire a composition close to that of diamond.

The official category, Iron Snake Pokémon, sums up both ideas. It closes a two-stage evolutionary line with no middle stage, and it doesn't happen by levelling up: you have to trade Onix out of the game for it to come back as something else.

The 200 Defense is the figure that dictates everything else. It's one of the highest in the Pokédex, and it comes with 75 HP, 85 Attack, 55 Special Attack, 65 Special Defense and 30 Speed, for a total of 510. In other words, almost nothing makes a dent from the physical side, while it folds ridiculously easily to special attacks. That 65 combined with 75 HP is the crack through which games are lost.

The typings help on defence. Steel and Ground leave Poison and Electric out entirely — zero damage in both cases — reduce Rock hits to a quarter, and halve a long list that includes Normal, Flying, Bug, Steel, Psychic, Dragon and Fairy. In return, there are four double weaknesses: Fire, Water, Fighting and Ground.

Sturdy, its usual ability, stops it from being taken out in one hit, guaranteeing it a turn even against the special attack that should finish it off. Rock Head, the other ability, cancels recoil from moves that cause it; none of the seven it learns by levelling up falls into that category, so in practice it does nothing. Its hidden ability is Sheer Force: it boosts the power of moves with secondary effects and removes those effects.

Its real job is to take hits and leave the field prepared. It gets Stealth Rock at level 25, and that's the most useful thing it does. For hitting back, it has High Horsepower at level 34, with 95 power, and Iron Tail at level 43, with 100. That Steel typing does its work against Fairy-, Ice- and Rock-type opponents; against anything that moves first and targets its special side, it won't make it to a second round.

It belongs to the Mineral Egg Group and no other, a club of rocks and gears that puts fairly tight limits on possible breeding pairs. The gender ratio is 50%, so getting a female isn't the bottleneck it is in other lines. Its 25 hatch cycles amount to roughly 6630 steps, putting it among the longer stretches.

It has no Egg Moves of its own and no assigned Move Tutor moves, so anything it knows outside its natural list has to be taught through other means. And that natural list fits into seven entries: Rollout at level 1, Tackle at 6, Bulldoze at 11, Rock Slide at 18, Stealth Rock at 25, High Horsepower at 34 and Iron Tail at 43.

Evolution is the awkward part. Onix evolves into Steelix by trading, and only if it's holding a Metal Coat. You need two players, or two games and a spare console, for something other species handle by gaining a level. The irony is that wild Steelix sometimes hold that same Metal Coat as an item.

In the wild, its habitat is caves, with a catch rate of 25 — well below the 45 already considered difficult, and a world away from the 255 of a first-route Pokémon. Defeating it awards 2 effort points in Defense, and it grows at a medium pace.