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!"

I think you're right about Flumphs, though. WOTC/Hasbro owns the copyrighted names of these things, but they're generally folklore, based heavily on someone else's IP whose estate hasn't enforced copyright, or too generic. So, they go the Games Workshop route of using original copyrightable names (like Space Marines vs Adeptus Astartes, it's Displacer Beast instead of Coeurl), so other companies can call them something different and execute their abilities different enough and probably be okay (if they can afford to fight it in court).