The rotwitched are natural forest animals (and some unnatural ones) which have stumbled into pockets of ancient magic. Like anything else organic, magic can spoil, fester, and rot, and when it does, its original intent becomes twisted and broken. Most magical effects fade away long before this happens, the mana used to power them running out long before it has a chance to turn. But when there is abundant mana available to a source of magic for a very long time, the magic spell, if not cast absolutely perfectly, can go wrong.
When a living creature stumbles into such a magical place, it can have dire effects upon the creature, twisting it, melding it with other creatures, polluting its nature with unclean magic. The form of the rotwitched are myriad, but they are always nightmarish in appearance and violent in demeanor, for the magic corrupts both the outside and the inside of anything it touches.