Invisible Pig was a Minion Pig who got turned invisible
Voice Actor: Sean Charmatz (voice)
A Minion Pig was chosen to be a test subject of Garry's Invisi-Spray as he shows that the Invisi-Spray works, but was shocked to hear that the spray lasted forever, so he called his girlfriend Tina to tell her that they will not be able to see each other ever again.
It is unknown how Invisible Pig will be able to live his life or what will happen to his life. My guess is that he committed suicide after the events of The Angry Birds 2 Movie.