Fairy does ×½ to Poison

Generations VI to IX, single types. The other way round, Poison against Fairy, is ×2.

64 Fairy and 83 Poison species among the 1025 this site covers.

Attacker, Psychic and Fairy Hatterene sprite

damage×½Generations VI to IX

Defender, Poison and Ghost Pecharunt sprite

Fairy did not exist in Generation I and Generations II to V, so Fairy against Poison has no value there.

Strongest Fairy attackers

Hatterene leads Fairy at Sp. Atk 136

Sturdiest Poison defenders

Dual types that change the answer