This manmade Pokémon evolved from the latest technology. It may have unprogrammed reactions.
– Pokemon Crystal Pokedex
Porygon2 looks like a smoother, upgraded version of Porygon (hence its name, the 2 being version 2 like a computer program). It’s more rounded than the pointy Porygon, and more duck-like.
Porygon2 was introduced a generation after Porygon. Its Pokedex entries say that sometimes it exhibits unprogrammed behaviour, suggesting that some data has been corrupted.
Stat-wise, since I think Porygon2 was originally supposed to be the final evolution of its family until Gen IV, it has pretty good stats. At bast stat 515, it has one of the highest stats of not-fully evolved Pokemon. A new held item introduced in Gen V, Eviolite, can be very useful for Porygon2, because Eviolite raises the holder’s Defense and Special Defense by 50% if the holder is capable of evolving. With good Sp Attack, and boosted Defense from the Eviolite, Porygon2 can probably hold its own against fully evolved Pokemon.