I think a lot of people get into trying to identify all the species, sub species, mixed species, etc. of hogs to relate to behavioral characteristics. I can only relate to the large number of them I've dealt with in Texas, but they are probably a mix of lots of species. You won't see much of the short nosed domestic type here as most of that has been bred out of them, but every color a domestic pig comes in is represented in the woods. A person can look hard enough to find agressive instances and I would guess most of them are either initiated by people and the hogs are trying to defend themselves or hogs getting confused about which way to run to get away.
Hogs do tend to get noisy during some of their daily activities, but most of it is not fighting. Something as simple as one bumping into another can get a grunt of squeal out of one and this is normal to them. A person seeing boars fighting is just a show of dominance such as is seen in almost any species...including some humans after a few adult beverages. A few disagreements can be seen around food sources as pigs jockey for the best chances at the food ( they have little for table manners and can make real hogs of themselves).
As far as european species making it into the U.S. there are reasons for this as well. Texas..as well as some other states...has a big market for exotic species and raises them for hunters. This is big business and people pay simply insane amounts of money to get a trophy to hang on their wall. You can find people here that buy and raise all kinds of exotic animals for this market behind high fence operations. Hogs are masters of escape and there is a good chance the hog with the ear tag could have come from one of these operations. A really gnarly old boar that looks russian or exotic and has a really large set of tusks could fetch a thousand dollars or more in a trophy fee from one of these game ranches. There are many exotic types of deer, antelope, goats, etc...roaming on private lands in south Texas now that started out on game ranches.
I see ads occasionally on places such as Craig's list here wanting to buy feral hogs over the 200 lb. mark as some hunting ranches are short of them in their area. Now you talk about hog noise...try moving a bunch of them somewhere they don't want to go and they will tell you about it.