It appears that these creatures were at one point humans, however it is difficult to imagine the conditions that could turn a human being into this disgusting animal.
Determining the species from which the Zone's rodents developed has so far proved impossible although it can be said with certainty that their ancestors were gregarious mammals. Reaching up to 40cm at the shoulder they are more mobile, aggressive and ravenous than the normal desert animal.
It seems that these creatures are responsible for the deaths of a large number of stalkers. Few of those who have seen one were lucky enough to survive the encounter and according to them bloodsuckers prefer damp places around marshes and subterranean areas.
Pseudodogs are about as different from mutated dogs as wild wolves are different from mongrels. The creatures mark and guard their territory, attacking those who enter without mercy.
Mutagenic processes engendered by radiation and anomalies have played a significant part in shaping these mammals: they have lost all fur in places and grown long, bristly fur in others.
Like many other living creatures, domestic pigs in the Zone underwent serious mutation, which affected genes responsible for their metabolism causing the animal's phenotype to change significantly.
Truly supernatural invisible creatures that can be found only deep inside the Zone and live mostly inside ruined buildings. Nothing is known about their origins.