They key to creating a BoogeyMan is to never show his ( or her ) face.
If you want to make it have a face, encorporating animal characteristics into the creature would be cool.
Everyone has a different BoogeyMan, so it would be pretty hard to establish a universal one.
I think a black trench coat would be cool ,maybe vines coming out of some parts and weaving back in.
This would carry the man crossed with nature theme.