The Mutants are the previous residents of Pripyat, who did not follow the order to evacuate the city and have consequently become victims of the radiation released during the Chernobyl power plant disaster. The mutants have been exposed to radiation in such quantities that it has made their skin deteriorate and consequently become very slow-moving. The victims of the radiation (as the Ukrainian Special Forces referred to them) appear to want to cause harm to any and all healthy individuals, possibly to cannibalize them. This aggression to healthy individuals can be attributed to two theories: The radiation itself impacting the neural capacity of the mutants, thus causing them to not be able to control their actions, or because of constant isolation the mutants have endured has turned them against all uninfected individuals in a fit of revenge.
It is clear that the Ukrainian Government has known about the mutants in Chernobyl and Pripyat, but that they have 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 to be 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 began to roam the surrounding areas of Chernobyl and Pripyat.