At school we were read a book in 1975 ish, plus or minus a couple of years, the book may have been older.
Part of a small group (or couple) of children's larger adventure of discovery of their home-surrounds. We were also read Stig of the Dump (a classic from the early 1960s), but I'm unable to find references to the creature there.
The only elements that I remember are:
A bad-tempered boulder called "Nargon" or Naagon", some homophone-variant. This boulder chased children away from its territory. In the pursuit, the boulder became chipped. Somehow the children escape to safety, but the chip was still there with them. The chip scurries away into hiding.
There was an ancient, wise and cryptic creature there (friendly). The children are curious about the chip's ability to move, it explains "Any part of Nargon - is Nargon".
