A4 In the Dungeons of the Slave Lords was one of my early module purchases, and I found the myconids really creepy and interesting.
This post over on Old School FRPG (on Tumbler) explains why - they're mushrooms from Fantasia* turned sinister, as imagined by Erol Otus:
A lot of D&D is just "cute thing, done creepy."
* which I'll admit I've never seen all of. Maybe only the dancing brooms part.