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

I consulted with the experts - my cats. They say that both of you and
Samuel Redcat are correct about the terminology. (BTW they want to
know if Samuel really is a cat.) But they tell me that there are other
subCATegories known only to cats - greebers, greebets, greeblers etc.
I asked if they'd describe them but they refused. They say that
hoomins already know too much about cat affairs.