Subject line got my attention
First, the name: I'm N. Alex White - FF credits me as Neville White, but about 8 years ago I decided to switch to using my middle name as my primary name for various reasons. Anybody I've met in the last 8 years will only know me as Alex (so El-Remmen is in the clear!).
All of the creatures were originally submitted to the 'Fiend Factory' in White Dwarf; none of them were playtested, they were all just a matter of inspiration in one form or another. I've never owned a Fiend Folio (it was too expensive for me to buy when it came out!), so I can't be sure of the text of stuff, but I'll tell you how each originally came about, and what my 'inspiration' was in each case.
Stunjelly first.
Back in 1e days the 'clean up crew' were common dungeon hazards - the grey ooze, ochre jelly, black pudding, gelatinous cube and so forth. There were also a number of 'trap' monsters - ear seekers that assaulted those listening at doors, rot grubs that attacked those searching corpses.
As a DM, I'd noticed that whenever PCs were confronted by a long corridor with a dead end at the bottom, they would always go and search it. I theorised an evolutionary equivalent of the gelationous cube, although with the ability to mimic the colour and texture of nearby walls, silently and slowly sweeping along corridors, paralysing and digesting all that came into contact with it. I called this the 'living wall', it got renamed Stunjelly for copyright reasons.
Qullan
I liked the idea of mad raging tribespeople infecting others with their own madness, hence the short radius confusion effect that surrounded them. I similarly liked the idea of their insane insights giving them the ability to craft marvellously sharp non-magical weapons. But I didn't want anyone else getting those secrets, hence the feedback effect that kills them if they are charmed (which also gave a nice 'trick' for handling them too).
And in a metagaming move of champions, I guessed that there wouldn't be many entries starting with 'Q'
Shadow Demon
My favourite. I drew a picture one evening of this night-black demon, with heavily clawed forelimbs and hind limbs that was equally at home on two legs or on for. It only had small wings so it wouldn't have true powered flight, but could make prodigious wing-assisted leaps. It was actually composed of shadow, so it's power would wax or wane according to the prevailing light conditions. Finally, it had the ability to merge itself with someone and possess them, because I've always liked that kind of thing.
Although I sent my picture in, to White Dwarf, it appeared with a completely appallingly awful attempt by 'Polly' who did a lot of the illustrations in those days - she completely misread it. It had a much better illustration in the Fiend Folio, and if the Book of Vile Darkness picture of a Shadow Demon is 'my' one too, that is an excellent rendition.
In truth I've got no idea how much my original text was edited in each of the three cases - it was over quarter a century ago, after all!
I certainly killed a couple of party members with the living wall (aka stunjelly), I can't remember whether anyone met qullans and I know that although I placed a few Shadow Demons, nobody actually encountered them... to date
Any help?