Porygon made its first and only major anime appearance in EP038. This
episode caused almost 700 children in Japan to have seizures due to a
flashing strobe effect, and was therefore banned and never aired again
anywhere in the world. Since then, Porygon and its evolutions have
never played a main role in another episode, despite the fact that it
was actually Pikachu who caused the strobe effect which gave children
seizures.
Although it did not appear, a Trainer was attempting to send out their
Porygon to fight the Diglett and Dugtrio in Dig Those Diglett!. This
was only present in the dub. A Porygon made its dub debut in a brief
appearance during a graphic display in A Chansey Operation. A Porygon
made a cameo during the "World of Pokémon" opening narration of the
fourth movie, oddly being the only Pokémon in the shot appearing
behind a wire fence. It also had a similar cameo appearance in the
fifth movie as the "World of Pokémon" segment from the fourth movie
was reused.
http://bulbapedia.bulbagarden.net/wiki/Porygon_(Pok%C3%A9mon)