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 (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. 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. Garbodor represents trash pollution, Weezing represents air pollution and Muk represent sewer 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.
Famous Poison-type Pokémon Trainers include Koga, fifth Gym Leader of Kanto and now second member of the Indigo League; Janine, Koga's daughter who succeeded her father's position in Kanto; Agatha, the then-third member of the Indigo League (her main type is Ghost, but all of her Pokémon are Poison-type as well); and Roxie, second Gym Leader in Pokémon Black 2 and White 2. Also, most villain teams in the games specialize in Poison-type Pokémon, along with Dark-types.
They're strong against the Fairy-type because fairies represent purity and poison the pollution.
They're strong against the Grass-type because trees and plants are damaged by the pollution and toxins, such as insecticides.
They're weak against the Ground-type because earthen and sandy features neutralize and mitigate the toxicity of poisons, making them less effective.
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.
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 poisons 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, toxins that killed bugs. However, since Generation II, the Bug type takes normal damage from them because many insects have developed resistance to some insecticides.
The Ghost-type resists them because you can't poison someone who's already dead, besides, the spirit doesn't have a physical form, thus toxins can't pass through.
The Rock-type resists them because since rocks aren't live creatures, poison can hardly damage them.
The Steel-type is immune to them because poisons cannot circulate through or penetrate metals, mainly because they're not organic materials.
There are no Poison-type Legendary Pokémon that aren't an alternate form.