An unliving creature is a creature that, either by evolution or by some strange way of being, has their spiritual polarity flipped. If the base creature was damaged by positive energy, such as Cure and Heal spells, and healed by negative energy, such as Inflict and Harm spells, then the unliving creature is healed by positive energy and damaged by negative energy, and vice versa. An unliving creature's statistics are no different from a normal member of its' species except in how it reacts to positive and negative energy.
Note: Often the personality, outlook, and desires of the creature will be reversed as well as its polarity. In these cases, the creature becomes an outcast as it struggles against its' fellows. If this is the case for the creature, reverse the alignment part of the creature's stats(good to evil, law to chaos, evil to good, chaos to law, neutral to neutral).
I used this in a campaign to screw with the players after a large cataclysm involving negative energy.