It seems that these creatures were humans but it is hard to imagine what conditions could bring human to such state.
Snorks are crazy creatures which have animal style of life and they don`t differ much form wild monsters of the Zone. They move on four extremities, jump over land and smell it all the time to feel the smell of the prey. They hunting very careful and prudent and they wait for stalkers like wild animals. Lightning reflexes and hypertrophied muscles allow to do distant accurate jumps and to tear to pieces its prey.
Sometimes they have some details of army clothes and ammunition on their body and that is why we can suggest that they are missing stalkers.