So, what is the Poison-type?
Most RPGs have some sort of Poison status, but few form a true Poison element. Nonetheless, due to its status as a status, in most JRPGs Poison is treated as if it were an element. Pokémon has went further and made it its own element, and this inspired other RPGs which have not tried that to do the same.
In its original conception Poison was meant to be an all purpose type, accessible to many species through attacks and vice verse, but that was not realized in the first generation. Part of this was due to Poison's association with Grass and Bug, which made it initially part of Grass-type in TCG. Part of was due to lack of Poison-type moves. Nonetheless, Poison-type served as the filth associated type and the type associated with underhanded tactics until Dark was created.
In this generation, Poison-type's main purpose seems to be the "evil" type, that is, the type that is commonly associated with the villain team. Because there was no Dark or Shadow type in this generation to fit for underhanded tactics, Poison had to fill in that role. Poison also served as a low level coverage type, but because the type had so few moves, that went unnoticed. However, Gen I remains the generation with the most and most iconic species of the type, as well as the most useful. Everything else is a downfall for the type, until Fairies come.
Gen II was a mixed bag for the type, but largely it was a joke. While primary Poison-types were not hampered much by the Special split, secondary Poison-types were nerfed massively due to change, and have not recovered from that massive nerf. In addition, the species introduced in this generation were hilariously limited and terrible, Crobat being the sole exception. However, this generation brought the most significant Poison-type move, Sludge Bomb, a Psychic clone with higher effect chance.
Introduction of Steel-type has also nerfed the type significantly, rendering species that had no way to break out from the type useless.. Dark-type was initially a mixed bag, as some Dark-types gained Poison-type moves and vice versa, but largely Dark-type took over Poison's role as the dedicated evil type, which reduced Poison-type to the level of Bug-types, being a vermin.
Gen III tried to fix the Poison-type by giving it some iconic designs, but these have failed because they had terrible stats. The two new moves introduced in this generation, Poison Tail and Poison Fang, were cool on paper and terrible in execution. Not only that, but Dark-type was back, and replaced Poison's role in the new teams' parties significantly.
New Abilities given to Poison-types were largely bad, with Liquid Ooze being a good ability that was too situational, Poison Point being too weak and annoying, Stench being useless in this generation and Sticky Hold being irrelevant for now.
Gen IV tried to fix the Poison-type by bringing it back to basics. Team Galactic made liberal use of Poison-types over Dark-types, Black Sludge was introduced, several powerful Poison-types including Roserade was introduced. Physical-Special split brought in Poison Jab, a common coverage move was introduced, Gunk Shot was introduced and became a tutor, and Cross Poison was promoted in the pre-release period. There wasn't much new Abilities for the type besides Stench getting a flinching effect.
These buffs were not sufficient enough to remake the type, as Poison-type still suffered from lack of offensive effectiveness, but it made the type more efficient as a Defensive type.
Gen V was largely experimental with the type. This generation introduced some gimmicky moves like Coil, Venoshock, Sludge Wave, Acid Spray and Clear Smog to turn Poison-type into Psychic-type. This was because in the previous generation Poison was now associated with Psychic and Ghost-types, so it now had to behave like them. Because of this, Poison-type did not get much new species and Abilities.
The latter half of Gen V promoted Poison-types rather heavily, not because they were now relevant, but because the companies wanted to promote the new Gym Leader Roxie and the species associated with her. In addition to this, Poison-types were brought in to fill in Unova's new Pokédex. Nonetheless, with the coming decline of Fighting-type Poison-type's fortunes as a defensive species was all but null.
Gen VI continued the experimentation with the type. Only three species were granted Megas, which were largely jokes. Two new experimental moves were introduced, Belch and Venom Drench. Only one line was introduced, Skrelp-Dragalge. However, this generation further diffused the type's moves into other groups, particularly Sludge Wave for Water-types.
Biggest updates for Poison-type came from two fronts however, one being the stat buffs and the other being the Fairy-type. Fairy-type alone made Poison-type's offensive role to shift from pathetic to decent. With the introduction of Fairy-type, many Poison-types were buffed alongside it, and raised in usage. This was temporary, however, as Steel slowly came back as the dominant Fairy-killer.
Gen VII was a massive hit and miss. The first Poison-type legendary which was not a true legendary was introduced. Another major Poison-type expert that wasn't a true expert was introduced. Two new Poison-type lines with relative power and a new form with relative power was introduced. Few signature moves were introduced, one of which belonging to a pure Water-type.
Poison-types still remain largely experimental, however, despite some powerful new species. It is hard to say when they will have a true revival, but before that happens we will need to see massive buffs to existing species and some moves.
The Overview of Poison-types:
Poison is an odd type. Despite being meant to be a villain's type, it is actually quite defensive and not quite offensive, but its offensive options have been granted with nifty abilities, which make it an attractive coverage type. One might falsely assume that Poison's destiny was shafted due to its untimely meeting with Bug and Grass, two types designated to be the worst of the type chart, but the reality is Poison-type was always meant to be a bad type, just one for more oriented for mid-game. This is also partially why in the later games there is less Poison-types, as the mid-game is now dominated by other types, like Fairy and Ground.
Poison would need to make a full transition to an early route type in order to become a sufficient type. Otherwise, the anemic introduction of new species will not allow the type to gain future type experts and legendaries, and this will further flung Poison into obscurity. The only other solution would be to wait for another industrial region to introduce new Poison-types, as what happened with Sinnoh, Kanto and Unova.