So just what was the girl?

Based on my long-winded zombie body language analysis here...
http://www.imdb.com/title/tt0896534/board/nest/213014260

and my "zombies are real" history lesson here...
http://www.imdb.com/title/tt0896534/board/nest/249753407

I'm saying she's a feral mental patient who can't die. So, not a "zombie" by either common definition. a) a mindless, purely instinct driven, violent, brain-munching monster, or b) an unfortunate victim of a voodoo sorceror/slaver.

Neither.

I think in the simplest terms she was a medical experiment gone wrong. Whatever they were trying to do didn't work, and they couldn't kill her, so the kept her chained up in the deepest, darkest corner of the mental hospital.