Dungeon Mobs introduces a variety of hostile creatures and monsters. Instead of providing new animals that liven up the overworld and improve ambiance, this mod introduces a variety of new hostile mobs, all of which hate you.
However, it's not just enough that they hate you. Each of the mobs in this mod presents a different challenge, requiring you to think tactically about how to surmount the difficulty they pose. While the old stand-by of "hit it with your sword" may work for some, that may not always be the best - or even viable - solution.
You can find more information about the mod here.