Mythology and Folklore
There have been many creatures found in mythology and legend that describe creatures similar to Jack. These have been recorded throughout history. From different versions of the bogeyman, with some possibly based on early encounters with Jack.
In the countries of the eastern Mediterranean, children who misbehave are threatened with a creature known as "babau". In Italy and Romania, the Babau (in Romania, Bau-bau) is also called the l'uomo nero (Romanian: omul negru) or "black man". In Italy, he is portrayed as a tall man wearing a heavy black coat, with a black hood or hat which hides his face. This is startlingly similar to eyewitness' descriptions of Jack.
A mythical animal-like creature of Serbian mythology that hides in dark places, holes or abandoned houses, waiting to grab, carry away and devour its victims. I has a clumsy gait and is easily scared away by lights and noise.
In Germany the it is known as the "Buhmann" or the Butzemann. "der schwarze Mann" (engl. the black man), which refers directly to some inhuman or rather paranormal creature. In the Netherlands, the Boeman is portrayed as a creature that resembles a man, dressed completely black, with sharp claws and fangs. Minus the fangs this almost exactly describes Jack.
Unnamed Haitian Creature
There is a popular belief that a tall man, with legs 2 floors high that walks around the towns at midnight to catch and eat the people that stay outside.
The DC Comics character Ripjak which was cloned from the DNA of Jack the Ripper and bears an almost exact appearance of Jack served as inspiration for Jack's creation and appearance.