This is the creature I was fearing when I was a child. My grandmother used to tell me about these monsters. They were normal humans, but born with 2 rows of teeth. When they died they had to be buried face down, because otherwise they would chew their way out of the grave... And become undead monsters! When you are aged 5 and someone as reputable as your gran tells you this - of course you belive. There were other monsters she told me about too but this one was the most memorable one.
Although there are versions that they had two hearts and the second one would start beating when the person died. This is one I decided to draw :)