The zruty is the only member of the z zruty monster class.
Zruty get two hit attacks and a bite attack. They have no other special abilities.
The nethack origin of the Zruty is not yet certain.
A book called The Mythology of All Races lists the zruty with this quote: "The Slovaks have their Zruty, or Ozruti, who are wild and gigantic beings, living in the wildernesses of the Tatra Mountains." The similar phrasing suggests that this book is the origin of the monster and of the in-game Encyclopaedia entry. The Tatra Mountains exist in the border of Poland and Slovakia, so the origins of zruty could indeed be in Polish or Slovakian mythology.
The word "zrut" (correctly žrút, where ž is pronounced as French je and ú as oo in mood) means someone or something that eats fast and much. The plural form is "žrúti" or "žrúty". As such, a definitive place in mythology is still currently unknown.
The word "ozruta" would be probably best translated as "giant" (although a Slovak-English dictionary suggests also "ogre" or "whopper"). Today it is used to describe anything too big (often something that is bigger than needed, but also large mountains can be called ozruty).
The zruty are wild and gigantic beings, living in the
wildernesses of the Tatra mountains.