Acidman has a post touching on wild hogs. I've never run into one, but I know folks who have, and they have no fond memories of them. According to my dad, domestic pigs that get out and go wild, by the third generation, are totally feral, including growing tusks and a seriously bad attitude.
There's a lot of them in eastern & southeastern OK, and they do a lot of crop damage. About two years ago a game ranger checking out some damage had his leg ripped by a boar before he could kill it, and I've heard of a number of other close calls.
A guy named Humberto Fontova has written a couple of books about fishing and hunting in Louisiana, The Helldivers Rodeo and The Hellpig Hunt. Apparently some actual Russian boar got loose in the bayous down there and made themselves at home. I don't think I'd like to meet one of them unless I had a 12 guage and slugs, they are seriously big and tough.
Peter Hathaway Capstick was a wonderful writer, mostly about hunting and fishing in various countries. He wrote a piece- originally a magazine article- I read in one of his books about hunting the Russian boar that were imported into Argentina. Big, powerful, fast beasts with a taste for killing and eating sheep, and some of the locals hunt them with knives. (no, not on the best day I ever had!).