These Pokémon have a natural toxic quality; some directly represent real-world species known for their venom, such as snakes, and some even represent pollution itself. They normally live in caves, marshes or similar places.
Most Poison-type Pokémon are based on poisonous or venomous animals (Pokémon such as Seviper, which is a snake, Drapion, which is a scorpion, and Scolipede, which is a centipede). Some of them are also based on poisonous plants or fungi (Pokémon such as Foongus, which is a mushroom, and Oddish, which is a mandrake root). Most dual Poison-type Pokémon have Bug type or Grass type as their other type. This reflects how, in real life, many insects and plants are poisonous or venomous. A very curious fact about Poison-type Pokémon is that some of them resemble ninjas. For example, Koffing is like a smoke ball, Crobat has a ninja's speed and secrecy and Toxicroak has paws that make it resemble a ninja. Some Poison types also resemble types of pollution. For instance, Garbodor represents trash pollution, Weezing represents air pollution and Muk represents liquid pollution. Strangely, not all Poison types are based on poisonous or venomous things (Stunky, which is based on a skunk, and Skrelp, which is based on a leafy sea dragon, are both real-life animals that are not poisonous or venomous).
The Poison type is related to the status effects Poisoned and Badly Poisoned, which are produced mostly by Poison-type moves.
They have no effect against the Steel type because poison can't circulate through or penetrate metals, mainly because they're not organic materials.
They're strong against the Fairy type because fairies represent purity and poison represents pollution, and thus fairies have weak immune systems.
They're strong against the Grass type because grass represent nature and poison represents pollution, and nature like trees and plants are damaged thanks to pollution and toxins, such as herbicides.
They're weak against the Ground type because earthen and sandy features neutralize and mitigate the toxicity of poison, making them less effective. Alternatively, it could be because while living organisms and thus the Earth's biosphere may be devastated by pollution, the Earth itself is relatively unchanged by it.
They're weak against the Psychic type because mind can't be poisoned. Poison acts over the body, and the mind is superior to the body, while Psychic-type attacks are mental. Alternatively, it may be because going into a medical field requires a lot of education and knowledge, giving the idea of keeping poisons at bay, hence the term "mind over matter".
They resist the Fighting type because when poisoned, a fighter or deportist loses strength. Fighters try to be healthy as most as they can and poison can make them sick. It could also be said that since some Poison-type Pokémon have a gelatinous body, punches and kicks cushions the impact. Curiously, the Fighting type isn't weak to them.
They were strong against the Bug type in Generation I because of insecticides, which are chemicals used to kill bugs. However, since Generation II, the Bug type takes normal damage from them because many insects have developed resistance to some insecticides and sometimes even carry toxins themselves, rendering many poison-based bug attacks such as insecticides will be less ineffective, thus making it the reason they resists the Bug type.
The Ghost type resists them because the spirit doesn't have a physical form, thus; toxins can't pass through. Additionally, if you try to poison a corpse, the corpse won't be affected because it's already dead.
The Rock type resists them because since rocks aren't living creatures so poison can hardly damage them.
They resist themselves because venomous animals of the same species can't poison each other. However, there are other real-life factors that contradict this resistance. If a venomous animal of a different species injects their venom into another venomous animal's body, that animal, even if they have an immunity to their own venom, may be affected by the other's venom.
Trivia[]
Despite being all physical attack moves before Generation IV, there are more Special moves than Physical moves that are Poison type.
The only type combination that allows for a double weakness to Poison type is Grass/Fairy.
Strangely, the Water type takes neutral damage from the Poison-type instead of being weak to them, despite the fact that water is very easy to pollute, especially by toxic waste, and aquatic beings have great difficulty surviving in polluted water.
Currently, all of the Generation I Pokémon with unique type combinations that are not Regional forms are Poison types.
No even-numbered generation has introduced a pure Poison-type Pokémon.