As much as urban legends are usually 80% bull shit and 20% true, this one has so many details that it creeps you out just to think about it.
In June of 1976 a weird woman was brought to the Cedar Sinai hospital in Los Angeles. The fact that she was holding a cat in her mouth was one thing that qualified her for weird. But it wasn't the most freaky thing about her. She was wearing a white dress, now covered with the cat's blood. She didn't want to let go of the feline. Something about her features made her look inhuman. She looked like a mannequin - expressionless and with her eyebrows missing (maybe shaved).
After removing the cat from her mouth, the doctors noticed she was unusually calm. She didn't seam to show any sign of emotion. The doctors decided to give her a sedative and that's when she freaked out. If you believe a witness, she was still expressionless when she struggled with the doctors. At some point she gave one of the doctors a grin and he let her go - either out of surprise or fear. The witness claims she had spiky teeth like an animal. She bit the doctor in the neck and fled.
Nobody saw her ever since and the doctor bled out. The witness is a nurse that helped him hold the weird creature and the experience affected her mind. The creature killed 3 people that day - the doctor and two other men who were unlucky to be in her way out. They received severe bite wounds that caused them to bleed out.
This story would be something like from a movie if it wasn't well-documented. There's a bunch of people who saw her being admitted (she was an unusual sight). The police had no idea what to do with the case. The official version was that the woman was some kind of a psychopath that sharpened her teeth and acted feral. The nurse that survived claims that it didn't look or act human. It was more like a mannequin and only resembled a human being. Her moves were also very... awkward. She is said to bend her neck in weird angles while looking at the people around her during the moments she was calm.
What the hell was it? I guess no one there figured it out, and it wasn't seen in public for over 40 years. The best bet would be either a demon (like the nurse claimed, among other weird stuff) or it actually was some weirdo that got away from someone's house. Maybe somebody made her like that - the sharpened teeth and shit. Maybe it turned someone on to raise a little monster. Who knows?
There's a photo of the woman from the story but I'm not sure whether or not it's actually her or prepared as an interpretation (there's no sign of the cat's blood on the dress, but perhaps it's scrubs they gave her at the hospital; it resembles the ones the doctor and the nurse are wearing on the photo).