Characteristics
The aufhocker is never identified in literature as having an identifiable shape. It's known for taking on the form of animals and in some cases, human beings in order to fulfill whatever duty it sees fit. In most cases, the aufhocker is considered to be a very dangerous theriomorph that tears the throats out of humans. The connection to attacking victims in the throat is what links the aufhocker to vampirism.
The aufhocker cannot be killed. In vampire mythology, sunlight and church bells are often used to frighten vampires. These defenses are also linked to the aufhocker.
Legend
The name aufhocker means "leap upon." This is because according to legend, the aufhocker would leap upon the back of the victim before biting their throats.
References
Aufhocker Wikipedia(Text) CC BY-SA