It's interesting how the prophecies depend on how you interpret them. In this case, the prophecy was that "something dangerous would appear" and it was right; those two magical boars were dangerous. But by hunting them, the village gains a lot of meat to feed everyone. The so-called "negative" prophecy ended up to be good because the right actions were taken. This means prophecies aren't positive or negative, but just tells them as it is. Those boars are dangerous but what they do with it will determine whether they get lots of food or the village gets destroyed.