Porygon-Z is a modification of Porygon2. Porygon-Z appears similar to Porygon2. Most notably, its head is detached from the body and upside down, much like Porygon's legs, with no visible neck and yellow eyes with a dartboard pattern. Its bill is more round, and doesn't curve down as it did in its Porygon2 stage. Its legs are now wing-like, and its posture resembles that of a hummingbird. The tail is a little plumper, with the blue pattern on its chest no longer being isolated and now connecting to its legs/wings
*taken from Bulbapedia*