pet rock

plural pet rocks

1. A rock one keeps and cares for as if it were a pet, often purchased as a novelty item and jokingly treated as having needs like those of a living animal.

   I made a cute family of pet rocks by putting eye, nose, and mouth stickers on big stones.

   2 more examples

   Though the fad ran its course long ago, the phrase “pet rock” endures in the American lexicon, denoting (depending on whether it is uttered with contempt or admiration) a useless entity or a meteoric success.

   [2015 March 31, Margalit Fox, “Gary Dahl, Inventor of the Pet Rock, Dies at 78”, in The New York Times, →ISSN

   Papyrus: This is my brother's pet rock. He always forgets to feed it. As usual, I have to take responsibility.

   2015 September 15, Toby Fox, Undertale, Linux, Microsoft Windows, OS X