The status-spreading specialist — and Fairy's only counter.
Poison spent most of its life as a support type. Then Gen 6 added Fairy, weak to Poison and Steel, and suddenly Poison mons had a real offensive niche again. Toxic Spikes, Sludge Bomb and Gunk Shot became essential tools for any team that needed to check Fairy. Meanwhile, Toxic itself — a status condition that doubles damage each turn — is one of the oldest stall conditions in the game.
Poison is a hazards and attrition type. Toxic Spikes on entry hazards wreck grounded switch-ins. Black Sludge (Poison-exclusive held item) passively heals Poison holders 1/16 per turn. Regenerator + Poison typing is one of the best wall archetypes in the game (see Amoonguss, Toxapex).
Sludge Bomb (90 BP, 30% poison) is the special nuke. Gunk Shot (120 BP, 80% accuracy) is the physical nuke. Toxic Spikes layers up on entry. Baneful Bunker is a protect move that poisons contact attackers.
Nidoking was the first Pokémon ever created, not Bulbasaur or Rhydon — Ken Sugimori confirmed in interviews that Nidoking was the initial prototype design that established the art style for the entire Red & Blue roster.