Enchanted Creatures

Trolls, giants, and other creatures that practice the old magic sometimes take beasts and other lesser beings as slavish door-wardens or guardians. Often, they will enchant the chiefest of their beasts, ripping them open and inscribing runes into their bones. Much as it does to weapons and wands, it ingrains the magic of the runes into them, transforming them into something more.
Many of these beasts, their masters slain in war, now roam, and pose a challenge to even the most worthy of warriors.