The human mutants had been exposed to radiation in such quantities that it had made their skin crack and become very slow moving. The victims of radiation (as the military men at the end referred to them) seem to want to hurt unmutated individuals, possibly to eat them. This aggression to unmutated individuals can be due to two causes, either due to radiation poisoning or from the isolation these creatures experienced after them being exposed to radiation.
There were also other types of mutants protrayed in the movie, including the large, strange fish seen in the lake Uri "pulled a prank" on.
It is clear that the Ukrainian Government knew about the mutants in Chernobyl and Pripyat but that they kept it secret from the public and other countries except possibly from some russian military forces and officials. The mutants were originally "patients" being treated in what seems as a temporary, underground hospital/prison. The guards and doctors working there knew the mutants were a danger to the public and therefore kept them secure. However it seems like the "patients" escaped the treatment facility and started to roam the surroundings around Chernobyl.