The Aufhocker is a malevolent shapeshifting Black Dog from German folklore.
The name "Aufhocker" literally means "Leap-Upon", which is how this spirit attacks its victims.
Like the similar entities Kludde and Oschaert, the Aufhocker could take the shape of a monstrous Black Dog with burning eyes. It can also take the shape of a black horse or another domestic animal. In some stories it was the spirit of a wicked person who came back to life to terrorize the living.
As a black dog, the Aufhocker would follow human victims on the road before jumping onto their backs, claws outstretched, and forcing them to the ground. It would then maul the traveler to death or change its weight, crushing him to death. Sometimes it would stand on its hind legs, raising itself until it could reach a person's throat.
When it took the shape of a horse, the Aufhocker would trick the weary int getting on its back, at which point it would break into a run, not releasing the victim until they reached a swamp or pool of water. It would then pitch its rider into the water and disappear, leaving the victim alive but soaked to the bone.
The Aufhocker was a spirit and could not be killed, but there were ways to escape it. The creature only appeared at night, and if the victim survived until dawn broke the spirit would vanish, letting him go free.