He was born a human, but has been afflicted since birth with a “curse” he ignores the origin of
He can shapeshift into a piglin (a very small one), and this form makes him fitter and makes his senses more acute. Unfortunately, he cannot really control it. It shows up as a defense mechanism when hes feeling threatened (even if he tries to hide his emotions his body will show them for him! lol)
During season 7 he was finally able to shift into his piglin form, which is why he's able to escape so many death threatening situations now
He has a third form: a wereboar form that very rarely manifests. He cannot control himself while in that state, he feels as if he were dreaming. It shows up when he's actually about to die, meaning he's using strength he doesn't have. Once he's back in his human form he's extremely tired and needs a few days to get back in full health. It started to manifest during season 5.
Another thing about wereboar!Bacon is that, if you manage to calm him down, he will come to his senses and be able to think clearly. He can't talk, however; that form has no proper vocal cords, it's all grunts and oinks.