So, what are Poison-types, really?
The Purpose of the Poison Type:
The primary purpose of the Poison-type was to showcase the poison status. Outside of this, Poison was meant to be a type for the villain team and other annoying enemies one might encounter in the mid-game. As the franchise grew, Poison tried the role of a dual offensive type, but when that failed, it became a defensive type. Initially associated with Bug and Grass as a forest-based type, its urban character took over and it became associated with Psychic and Ghost instead.
Poison-type's mid-game status disallows it from getting powerful late-game variants, and this means that Poison on average has terrible BST. Because the variety is limited as well, most Poison-types become irrelevant after a generation, and despite the gimmicky moves the type has, it is far from a fully utilized type.
The Aesthetic Archetypes of Poison Types:
Most Poison-types are purple, though some are Black as well. Aesthetically, Poison-types can be divided into following categories:
These are species based on flora, fauna and other organic creatures that have poisonous elements. These Pokémon often have Poison-type as a secondary ability, but species whose poison is their primary characteristic, those become pure Poison-type. Depending on the type, they can be in Field, Bug and Water 1/2/3.
These are species that either represent pollution, or contain some sort of polluting or toxic material that is not a toxin, such as sulfur smoke or miasma. Pollution based species are often in Amorphous, while other toxic species are in relevant Egg Groups.
The In-game Archetypes of Poison Types:
Mid-game Poison: The most common archetype, these are species that are encountered in the mid-game. They are often in 480-500 BST range, and their pre-evolutions tend to be used by the evil team.
Early-game Poison: The second most common archetype, these are species that are encountered fairly early game. They are often used by the villain team, but sometimes they are used by irregular thugs.
Blob: An archetype that appears in odd generation games, these are two staged species with high HP and decent offenses, but generally weak overall stats. They range from 460-500 range.
The Stat Archetypes for Poison Types:
Defensive: Regular old walls. Some are dual defensive.
Sweeper: Regular old sweeper. This is a somewhat rare group for Poison-type.
Bulky Offense: Regular old bulky offense.
Stat Buffer: The conventional Poison-type offensive archetype, these are species with okay stats that rely on stat buffing moves to become a threat.
Annoyer: Regular old status spammers, these can be defensive or speedy in Poison-type.