The virtual Pokemon Porygon can be found on PokeStops located at public transportation terminals. This was the pattern observed by many Pokemon players, and it seems that the Pokemon Go eggs found on airports could hatch similar creatures.

Porygon is a very rare catch in Pokemon Go. It is a normal type Pokemon that has a virtual attribute and could live in computers or any digital means. As observed by Silph Road community member greyguy12 on Reddit, you can have a Porygon in two different ways - hatching an egg or catching it live (well that's basic). However, you must take note where Porygon could possibly be.

As mentioned, Porygon seems to be pretty fond of transportation terminals, either public or private. The first guy to have been reported to catch a Porygon needed to hire an Uber to follow the Pokemon's whereabouts. Then there follows other players who was able to get the rare catch.

As noted by all players who successfully got their hands on Porygon, all of them were on airports or ferry stops when they did. Also, Pokemon eggs that could hatch a Porygon can be picked up on these locations. There were five cases in which a Porygon egg was found on airports, and eight cases at a PokeStop near ticket counters for ferries.

During the early release of "Pokemon Go," Pokemon spawning on airports is not a thing - not until the November 2016 update where Niantic may have been rolling in Pokemon on those areas. This was intended to maximize the playing spaces for Pokemon Go.

Still, airports are pretty "private" in a way, so the claims of greyguy12 may just be a "coincidental assumption." Spawns on airports of specific types of Pokemon still hasn't been officially confirmed. But since there is no particular lead yet about Porygon's whereabouts except the "public transportation" thing, it wouldn't hurt to try if there is a chance.