I thought they were merely alternative terms for the same entity - that
is, the one that a cat will stare fixedly at or hunt ferociously, but
which cannot be detected by mere humans.

Cheryl

Samuel Redcat suggests that greebling, greeblie, and greeble can be used
interchangeably as far as hoomin use goes. He says the distinction
between them is a little more complicated than most hoomins are capable
of distinguishing.

Dan