Creatures-
“Hebi-ushi / Shambler”
The domestic livestock around Fukushima, abandoned in the disaster and evacuation, reacted badly to the radiation. Recovery teams frequently find mounds of vegetation waddling across the countryside in groups. They resemble cows, horses, and indeterminate crossbreads of livestock, covered in muck and seaweed which is actually part of their skin. Shamblers are not “aggressive” and definitely not carnivores (exactly what they eat isn't clear), but they are fiercely territorial and easily startled. But because they resemble plants, agents can easily find themselves walking through a herd on the verge of stampede without seeing anything more than thick shrubs.
(Hebi-uhsi would fill the role of both dogs and Flesh, but rather than actively hunting the player, they would wander through an area, hard to see, and then chase targets that get too close.)
“Kamome / Gull”
With the entire region turned to coastal swampland, the seagulls that orbited the local fishing industrial continue to survive. While the gulls don't appear to be physically changed in any way, they have indisputably become more aggressive. Birds will stay well away from any danger, but will swoop down to seize anything edible left in the open, and may attack wounded or unconscious people.
(No specific parallel to anything in Stalker, just ever present scenery like the crows, with the slightest hint of danger when they grab food off the ground.)
“Tako / Octopus”
When the irradiated water crested over Japan, it deposited huge populations of unwitting sea life far inland. Most just became so much detritus, but a mutant species of octopus managed to survive, and even thrive in the watery aftermath. The “Tako” are amphibians in the truest sense, capable of surviving for days out of water, but never straying far from it. Bigger than dogs, and considerably smarter if less aggressive, Tako drag themselves through the debris, naturally camouflaged, scavenging for anything to eat, living or not. Tako attack by waiting for an opportune moment, then leaping at a target, trying to latch on with their tentacles and bore through armor with their crushing beak. If successfully repelled, the Tako will try to escape into any dark hole it can find, to wait for another opportunity.
(The Tako lies between Bloodsuckers and Snorks, roaming anywhere near water, laying in wait for prey before attacking and running off again. An iconic image of Shores of Fukushima would be turning towards a gurgling scream just in time to see a huge tentacled beak flying at the camera. This would need some mechanics, like knocking the PC to the ground and wrestling the octopus off.)
“Oni / Ogre”
Recovery agents in the countryside and most devastated areas have encountered gigantic humanoids, hunting for food and living in crude shelters. They appear as bloated and overgrown humans at first, but dissection reveals these “ogres” to actually be multiple bodies fused and compacted into grotesque humanoid piles, that manage to stumble on two feet and grasp tools in their meaty hands. Although invariably human in shape, they can be made of almost any animals, and many have hooves and horns from absorbed cattle. It is not clear if ogres are assembled just at once or if they can incorporate more body mass over time. Both bestial and weirdly intelligent, some researchers insist they have seen ogres traveling in pairs or teams, and are even capable of communicating, but this has never been documented.
(No specific parallel to anything in Stalker, except maybe the Pseudogiant.)
“Kitsune / Fireball”
More legend than science, recovery agents in the thickest knots of the exclusion zone swear to have seen wisps of fire circling in the air, often orbiting or shielding one especially large plume that moves of its own accord. Sometimes these wisps will follow a person, bombarding them with thrown objects wreathed in flame. Other times, flames will manifest at a distance or in corridors, trying to lead people into a very quick death, or supply caches or nothing or anything. In the countryside, animals seem to totally ignore these fireballs, and some witnesses swear to see them act in concert.
(Combination of Poltergeists and Pyrogeists, with the flavor of the Red Forest Psy-Dog. Unpredictability is the key.)
“Kage / Waves”
Easily the strangest unexplained manifestation in the exclusion zone, if arguably the least dangerous, are the rumors of “ghosts” and moving shadows that follow people through ruins. Within buildings, observers have seen spots of negative-lighting,
light on shadow or shadow on light, in vaguely humanoid shapes that seem to waver in peripheral vision. Their correlation with strong but temporary pockets of radiation likely relates them to similar moving pockets of radiation seen outdoors, in hazy patches in the air. Normally just still if bizarre lighting effects, witnesses claim these shadows occasionally move around as if deliberately attempting to draw a person's attention, but fade away when watched intently.
(Obviously no connection to Stalker, but draws on the ghost-enemies from the Brain Scorcher, but more oblique. Aside from radiation, they'd have no way of interacting with the PC, but could hint at hidden enemies or useful paths.)