I honestly feel like certain Kanto Pokémon were given the Poison typing as a so-called “balance change” that failed in spectacular fashion. Poison’s in a rough spot because it’s weak to both Psychic, one of the best Types in the generation, and Ground, the Type that exists in large part to keep the otherwise broken Electric-Type in check. However, if we look at all of Gen 1’s best Poison-Types, you’ll notice they tend to be the ones with Poison as their secondary typing, not their primary typing. We know from the Flying-Type being exclusively secondary that most Pokémon are given these in a specific order for a certain reason (Example: Normal/Flying as opposed to Flying/Normal for Pidgey and Spearow) so the implication here seems to be that the Poison typing was added to some of these Pokémon later in development, possibly for the sake of giving the newly added Ground-Type more viable targets to hit for super effective damage.
This is an interesting idea for Poisons but it doesn't match the data. There are only three primary Poison-types, and two (the Nidos) are fantastic (for in-game, the balancing context that makes sense for RBY's release). Secondary Poison-types range the whole gamut from great (Gengar) to good (Victreebel) to unremarkable (Vileplume) to legendarily awful (Beedrill).
The secondary Poison-types feel unlikely to be a mechanical balancing choice. Many are deeply immersed in Poison flavor-wise, which is a more straightforward reason they got Poison. Tentacruel is a jellyfish, Victreebel is a pitcher plant digesting prey with corrosive acid, Vileplume is based on the corpse flower and spreads toxic pollen, with its pre-evolution Gloom being notoriously stinky. It'd be weird if these weren't poison. You have a 2/3rds chance to never battle an opposing Venusaur or member of its line, so balancing the game around fighting it seems dubious. Venomoth is more fringy, but it reminds us of something else. Of the 7 fully evolved secondary Poison-types, Ground only hits 2 of them super effectively anyway, Gengar and Tentacruel (who threatens Grounds and Rocks in return with fast special Water attacks). All the rest are also Grass- or Bug-type, so they're only neutral to Ground.
Occam's razor tells me that Gengar has Poison because Gastly does, and Gastly has poison because it's a poisonous cloud of gas, and Gengar's poison flavor just ended up weaker for whatever intentional or unintentional reason.
"What secondary type is the worst?" is an interesting question. Gen1 Poison is unique for providing almost no advantages, but at the end of the day, two weaknesses is unfortunate but survivable. If Gengar can't sleep Ground- and Psychic-types, it's unlikely to win 1v1 anyway, since it lacks STAB, is physically frail for Ground-types, and they take Thunderbolt / Psychic well, respectively.
Flying on some Bug-types might be worse. In earlier generations, they often lacked STAB, like Gengar's Poison. It adds three weaknesses, Ice Electric and Rock, which are all usually pretty common offense types, especially Rock in Stealth Rock games. Flying has resistances, but they synergize poorly with Bug's. It's immune to Ground and resists Grass and Fighting, but Bug already resisted these types anyway, so they get less benefit. The only novel advantage is a resistance to Bug, which is really not that useful, especially before Gen4 where Bug-types often lacked useful STAB.
I'll pick Gen4 Vespiquen as a specific one who hates being secondary Flying.
On defense, it's a bulky Pokemon that would really appreciate a helpful type chart to wall Pokemon, stick around, and maybe set up Defend Order. As mentioned, Flying gives two new weaknesses (a total of 5) and makes its Rock weakness 4x, which is especially horrible in Stealth Rock's debut, and only brings one novel resistance in the poor Bug. Access to Roost feels good, but you have Heal Order anyway. Your other Flying support moves are Defog, which doesn't clear hazards on your side yet, and Tailwind, which your 40 Speed self can't use very well.
Your Flying attacks are Air Cutter (requires Pt tutor), Aerial Ace, and Gust – your only Flying move by level. lol!
