"What non-D&D monster do you think is as iconic as D&D ones like hook horrors or flumphs, and why do you think so?"
Can I use Cthulhu?
I'm using Cthulhu anyway, bitches.
At this point, we should probably be arguing that some D&D monsters are as iconic as Cthulhu. He's headlining two major RPG core rulebooks, and is apparently also a cat now. The concept of subversive cults worshipping a dark entity while living out the rest of their lives without their nearest and dearest knowing they are batshit insane is a fantasy a lot of us have an easy time buying into, because holy shit the GOP am I right guys?
Joking aside, Cthulhu has appeared in such an insane number of media entities that he's transcended the popularity of his creator, H.P. Lovecraft. That's like people recognizing IT and not knowing who the hell Stephen King is.
Oh, and apparently some scientists named a spider after Cthulhu, because Californians never wanted to sleep again, anyway. Enjoy!