It was inspired by a creature called the coeurl.
You are correct: it was inspired by the coeurl, from A. E. van Vogt's 1939 science fiction story Black Destroyer, which was later made into the first six chapters of his 1950 novel The Voyage of the Space Beagle (1950).
From the word of Gary Gygax himself, dated January 26, 2005:
[...] the Displacer beasts I ripped off from the novel, Voyage of the Spaceship Beagle...if I recall rightly. That was one of the monsters in there - they sucked out something from humans with their tentacles, I don't remember!"
From the description, he can only be referring to the coeurl, the antagonist of Black Destroyer, which absorbs phosphorous (in the novel, potassium) from living beings. It is also described as having the ability to control vibrations, which the AD&D 1e Monster Manual describes is the origin of the displacer beast's displacement ability. To quote Wikipedia's summary of Black Destroyer:
A Coeurl, a large, intelligent, black cat-like animal, considers its near-future starvation as its food source of id-creatures has been hunted to extinction. [...] They lock him up, but the Coeurl's ability to control "vibrations of every description" allows him to easily open the electric lock.
And as per Wikipedia's summary of The Voyage of the Space Beagle:
Coeurl, a starving, intelligent and vicious cat-like carnivore with tentacles on its shoulders, approaches the ship, pretending to be an unintelligent animal, and quickly infiltrates it.
Gygax also confirmed the work as the displacer beast's inspiration in a forum post on February 12, 2005:
Q: Was the displacer beast inspired by the Couatl [sic] in Van Vogt's "Voyage of the Black Destroyer"?
A: It was Van Vogt's Voyage of the Space Beagle when i read it...but I suspect the story is the same, yes :D
August 2, 2007:
The displacer beast was most assuredly inspired by A. E. van Vogt's Voyage of the Space Beagle.
Gygax previously suggested it may have been inspired by a creature from P. J. Farmer's World of Tiers science fiction series, which at the time the displacer beast was published (1977) consisted of the novels The Maker of Universes (1965), The Gates of Creation (1966), A Private Cosmos (1968), Behind the Walls of Terra (1970), and possibly The Lavalite World (1977). March 10, 2003:
Displacer Beast--Vaguely inspired by a PJ Farmer critter in his "Created Universes" series, but not drawn from any specific thing therein.
However, it's possible he was mis-remembering in this instance.