A boar

[MarkD]

That's very interesting. Most dogs stand off of the pigs, barking and harrying them. Often this compels the pigs to move on. If a dog tries to grab a piglet certainly an adult will attack. Sounds like your friend's dogs ran into a pretty mean boar.

(But some local guys have hunting dogs trained to engage. Multiple dogs grab the pig; the hunter comes up from behind and stabs the pig with a big knife. Pretty ballsy.)