The description was a large six legged black catlike being with tentacles ending in rough pads rising from its shoulders. It was never quite where it appeared to be, weapons missing it as a result. (No not just hard to hit, but the creature appearing 'displaced'. Unlike the displacer beast the creature feeds off of the id as well as the flesh of its victims.
While the word displaced was used in describing the beast's ability at no time is it refered to as a 'displacer beast.'
The story is part of the book 'Voyage of the Space Beagle', available as an e-book from Amazon.com.