>>23368860>bunch of fairy pokemon have secondary typing that negate poison damageAssuming you also meant combos that are neutral to Poison, there are four. Two of them aren't worth using without their Megas and one of them is Carbink.
Poison is a genuinely good type to have on a defensive Pokemon. Good resistances, two easily managed weaknesses, a Toxic immunity, and a slightly more useful Leftovers in Black Sludge. Offensive Poison-types are more of a mixed bag, but they often have enough power in their high-powered STABs to make up for the shitty coverage. Game Freak has actually done rather well with making decent Poison-types.
Contrast to Ice. It's easily one of the coolest offensive types, but it has absolutely zero worth as a defensive type. So far, Game Freak has given us exactly one Ice-type Pokemon
that's not Arceus with base stats above 100 in both Speed and at least one Attack stat, and roughly 273K shitty "defensive" Ice-types.
