Rust Monster & Owlbear came from a set of cheap chinese plastic toys. Displaver Beast [coeul or whatever it was] stolen from a book. Purple Worm, I believe, stolen from Dune. Lich is just from RW folklore/legend.
Leaving mimic, mindflayer (though arguably stolen from Cthulu/Moorcock), gelatinous cube, and Beholder as those not stolen from anywhere, to my knowledge.
What is the most Iconic D&D creature from among those 4?
Beholder 31 - Oooogly as sin!
Displacer Beast 18
Gelatinous Cube 26
Lich 27
Mimic 23
Mind Flayer/Illithid 27
Owlbear 18
Purple Worm 24
Rust Monster 16
Beholder was stolen from the saying, "Beauty is in the eye of the beholder.". Beholders are ugly as sin, so he didn't even get the theft right.
I am aware that's the inspiration. I don't really consider taking a word out of a phrase and making something new whole cloth out of it as the same as taking nearly a direct copy of someone else's work and claiming it as something "new/yours" to be the same thing. So, no, not really a "theft" at all. As far as the ugliness of the beholder in light of the spawning phrase, that's what's called "irony." Again, not the same thing as filing off the serial number of something someone else came up with.