Grendel

A creature that only appears on hard or tougher modes (Or have the option to turn on). This creature will begin to stalk you if you survive (Without dying) over a certain amount of days. This creature is as fast as a horse, can jump several blocks high and is as tall as a wither skeleton. This creature is aggressive towards anything that attacks it or is good.   

Other idea: It lurks in all dimensions, but the normal and appears when players stay to long.

Other idea: If you travel more then 1 whole map within a given day/night there is a chance it may spawn in the distance or spawn in a village 

(Doesn't need to be the folk tale Grendel, and doesn't have to resemble the creatures look. I'm just looking for something to almost make me not want to travel alone.)