Porygon is the first artificial pokemon created that consists of entirely program coding. It is capable of moving freely into cyberspace. All Porygon's are copy-protected, and thus cannot be duplicated. It doesn't breath to survive, so reserchers are eagar to test it out in any environment.
Porygon's first appearence in the anime was Episode 38, "Eletric Soldier Porygon". In this banned episode, Ash, Misty, and Brock all go to a Poke Center to find a distressed Nurse Joy. There is something wrong with the poke ball transporter, which transports poke balls from poke centers to poke centers.
The pokemon she sends through are being turned into different pokemon. Akihabara-hakase the inventor of the transports system, explains that some pokemon theives have entered the computer world and caused chaos. He sends the heros into cyberspace to stop this where they soon meet Porygon.
This episode was banned from the whole world due to a scene that caused nearly seven hundred Japanese children to have seizures, irritated eyes, vomiting nd other symptoms due to a flashing strobe effect. The scene happened when Ash's Pikachu used an eletric attack on some vaccine missles, which caused an explosion of red and blue flashing rapidly.
4Kids Entertainment originally was going to dub this episode and lower the time of flashing or even cut some scenes out for safety, but was canceled as the Japanese government banned the episode. Originally there was going to be a New Year's episode for Pocket Monsters, but was never made after this accident.
Porygon never appeared in another episode again, and it's evolved form Porygon 2 never did either. However it can be noted that Porygon did made a harmless cameo appearence in the movie, Pokemon 4Ever.