Porygon is the first artificial pokémon 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 researchers are eager to test it out in any environment.
Porygon's first appearance in the anime was Episode 38, "Electric 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 poké ball transporter, which transports poké balls from poké centers to poké centers.
The pokémon she sends through are being turned into different pokémon. Akihabara-hakase, the inventor of the transports system, explains that some pokémon thieves have entered the computer world and caused chaos. He sends the heroes into cyberspace to stop this where they soon meet Porygon.
This episode was banned due to a scene that caused nearly seven hundred Japanese children to have seizures, irritated eyes, vomiting and other symptoms due to a flashing strobe effect. The scene happened when Ash's Pikachu used an electric attack on some vaccine missiles, 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 that was canceled as the Japanese government banned the episode. Originally there was going to be a New Year's episode for Pocket Monsters, but that 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 make a harmless cameo appearance in the movie, Pokemon 4Ever.