I know this is common with pot bellied pigs. If they have some small wacko traumatic thing happen to them in their nursing stage(such as mamma always wanting to roll over on em) The piglet can go all postal and will never really get socialized so its better to have a roast piglet because they will get mean in their adult life. Lotsa animals need careful socialization to avoid this. My neighbor has a hand of Belgian Draft horses and several "free range" pigs that wander the pastures with the horses. He also has a few pot bellied pigs he raises for his kids and others. He has always preached about early socialization and trying to keep them from imprinting something traumatic
As a kid , my dad raised Irish setters for show and field trials. We would begin handling them even before their eyes were open and it involved making fusses over them daily for extended periods of time.
The other dogs that I had raised were border collies and usually we only start socializing them after about 2 months. You want them to be developing their eye and abilities to handle others by just their stare and body. (they develop these traits very early)
Border collies that weve kept were always the one that didnt play like a kid but would sit in a corner and watch all the others. (Border Collies have never been mean in any owners Ive dealt with).
Sounds like some trauma got to the one pup. I guess you can treat it but it will take steady voice and a gentle hand on your part. Boxers from pounds often show fear aggression and it takes care to help them become good companions