Thank you all for your information on Utopce (that's how it's spelled?). That is actually very accurate. My father told me those stories and he isn't that old. He comes from a little rural (silesian) village and he truly believes those creatures exist. So I kind of know that my father must have gathered those myths from this region or even just this town.
But I still honestly, have only encountered this http://szkola.interklasa.pl/f014/stronawww/duchynie.html page on mamona wodic and Gniotka. It's a site about lasowiacy culture. I don't know what that is. They live in a region concentrated around Tarnobrzeg which is located in south-eastern Poland but as far as I know, my father lived in the western part of Poland. So how did he get those stories? Or are those regions connected?
For mamuna I've also found this: http://www.statemaster.com/encyclopedia/Mamuna
Either way, I've only looked into one book about slavic myths and haven't encountered any of them there. But if anybody can recommend me one…?