I would describe Shifties shift as a Boogeyman. The being can hide in small areas like closets and beds, can't be seen in sunlight, and can take many forms. All of which cause fear and dread in their victims. While you may consider the 'Boogeyman' to be a modern day phenomenon, there's a curious welsh prayer:
"From ghoulies and ghosties and long-leggety beasties and things that go bump in the night, good Lord, deliver us!"
For as long as man has existed, he has always feared the dark. Imaging dreadful things that go bump in the night that can take many shapes and forms, all of which are horrifying to who sees them. In the past some would call them Shades or Wraiths, today we call them 'the Boogeyman.' I would argue that the Boogeyman are shapeshifters as they take innumerable forms, in countless nightmares. Does it sound like something a emo-kid in Middle School would choose?
Well, yeah. But I'm in a rather ghoulish mood today. ;)
Of course, I understand if you don't want something that goes slither in the night and will instead search for something more traditional. And I'll get rid of the gun, after all, I still can get something using kind words. :p