Cleon
Legend
How about this?
Fast Healing (Ex): A tyerkow heals 2 points of damage each round so long as it has at least 1 hit point, its fast healing does not function in sunlight when it is not wearing a Human Skin (see below). If reduced to 0 hit points in combat it falls to the floor, at which point it is helpless. It regains 1 hit point after 1 hour, then is no longer helpless and resumes healing at the rate of 2 hit points per round.
Human Skin (Ex): A tyerkow can take on the guise of a living humanoid by putting on a skin. The tyerkow then appears just as it did when alive. When wearing Human Skin a tyerkow is not harmed by sunlight. There are rumours some tyerkow can strip the skin off their victims and assume their appearance.
Under weaknesses:
Leave in the bit about sunlight, but add a sentence "A tyerkow wearing its Human Skin is, however, not harmed by sunlight (see above)." Then add "Separating the Human Skin from a tyerkow and burning the skin (doing X hp of fire damage to it) destroys the tyerkow utterly." Not sure if we need the bit about water or not.
Looks worthwhile to me, although I'm thinking you should be able to burn a tyerkow to destruction while it is wearing its skin as well as without. Also, I'd quite like to keep the "destroyed at dawn" bit. Maybe something like.
A tyerkow wearing a Human Skin can be destroyed by burning it to ashes (reducing it to -10 or fewer hit points via fire damage). If a tyerkow's human skin is burnt while the monster is not wearing it (this requires 5 or more points of fire damage), the tyerkow will be forever destroyed at the next dawn.