These Pokémon have a natural toxic quality, maybe they produce venom, are poisonous plants, or even represent pollution. They normally live in caves, marshes or similar places.
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.
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 then second member of the Indigo League; Janine, Koga's daughter and new Gym Leader of Kanto; Agatha, the 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 videogames have Poison-type Pokémon, along with Dark types.