King’s Rock
Poliwrath in depth
Nyorobon, the angry tadpole
The English name combines polliwog, an old word for tadpole, with wrath. The Japanese name, ニョロボン (Nyorobon), starts from an onomatopoeia: nyoro nyoro is the sound of something writhing or wriggling, exactly how a tadpole moves, and the whole line carries it at the beginning. The belly spiral serves two purposes. Besides its anatomical explanation, it works as a hypnotic spiral, which is why it learns Hypnosis as soon as it evolves.
Four moves and a stone
Its level-up list is one of the poorest there is: Dynamic Punch upon evolving, and Body Slam, Bubble Beam, and Hypnosis at level 1. Nothing else. That gap is the direct consequence of evolving with a stone, because from that point on it stops learning moves by levelling up and its entire movepool depends on TMs and whatever it inherited from its Egg Moves. The practical lesson is simple: teach Poliwhirl whatever you want it to know before using the Water Stone, or at least be aware that afterwards you will have to rebuild its moveset from the machine list.
That list is broad, though. Earthquake, Focus Blast, Focus Punch, Earth Power, Muddy Water, Ice Beam, Work Up, and Drain Punch are all there, and with 70 Special Attack half of them are not worth using. The physical side is the one that matters.
The type that stood alone for twenty years
It was the only Water/Fighting Pokémon from the first generation until Keldeo arrived in the fifth. Four generations without company in a combination that now has several users, including Ludicolo in the Water/Grass variant with the same rain-based plan. In the sixth generation, its Attack was raised, one of the adjustments Game Freak handed out to veterans that had fallen behind, and its base total rose with it.
The other date that changed its life was the second generation. Until then, using a Water Stone on Poliwhirl was the end of the road, with no alternative; with the King's Rock and trading, Politoed appeared, and with it came a choice. Politoed got the Drizzle ability, which sets up rain on its own, and the comparison has been constant ever since: one summons the weather, the other takes advantage of it. A team that wants both needs two Poliwhirl.
Where it fits in a regular playthrough
Its real strength is spread-out bulk. With 90 HP, 95 Defense, and 90 Special Defense, it can take hits of any kind without specialising in one side, and Water Absorb turns half the aquatic Pokédex into harmless opponents. What holds it back is its Speed of 70, which leaves it moving after almost anything that can run in a story playthrough, and the lack of recovery that does not involve sleeping for two turns.
Against the Rock- and Ground-type Pokémon that fill the caves in every region, Waterfall and Close Combat handle most battles in one turn. Against Grass- and Electric-types, it is better not to switch in.