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 zruty are wild and gigantic beings, living in the
wildernesses of the Tatra mountains.
Zruty do not appear to exist outside of NetHack. As referenced in the Encyclopaedia entry, the Tatra Mountains, however, do exist in the border of Poland and Slovakia, so the origins of zruty could be in Polish or Slovakian mythology. Following on from this concept, on this web forum, someone has posted what appears to be the full text of a book called Vampires of the Slavs, which includes 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 description.
According to a discussion from r.g.r.n. The forms "ozruty" and "zruty" are in fact plural forms with "ozruta" or "zrut" being the correct singular forms. The word "ozruta" would be probably best translated as "giant" (although 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).
Word "zrut" (correctly žrút, where ž is pronounced as French je and ú as oo in mood) is someone or something that eats fast and much (dictionary translates it as "eatard"). Plural form is "žrúti" or "žrúty" depending on whether it is animate (žrúti - people that eat much) or inanimate (žrúty as in žrúty času = "time consuming (activities)", although also here it is often used with -i). As such, a definitive place in mythology is still currently unknown, because the word has no exact mythology tied to it. You can imagine "zrut" as a generic forest monster which just happens to eat a lot.