https://www.themoviedb.org/movie/11621
A pig's got to fly.
In Italy in the 1930s, sky pirates in biplanes terrorize wealthy cruise ships as they sail the Adriatic Sea. The only pilot brave enough to stop the scourge is the mysterious Porco Rosso, a former World War I flying ace who was somehow turned into a pig during the war. As he prepares to battle the pirate crew's American ace, Porco Rosso enlists the help of spunky girl mechanic Fio Piccolo and his longtime friend Madame Gina.
Fantastic movie, one of the underrated Ghibli ones! Great animation and storytelling! Also, I love that while there's some vague "oh it's because he was disillusioned with stuff" in outside documents, they basically never say in the movie why he's a pig. Everyone just kind of accepts it.
He said he has a curse, I think, but there's not much made of it.
He was a human before, he became a pig after a battle where he was the only survivor IIRC