Mannahnin
Scion of Murgen (He/Him)
Plausible, but remember that Greyhawk doesn't tell us they can't wear metal armor.No. They were monsters before they were a pc class (0e D&D Greyhawk) and they were trying to make fantasy magical Celtic cultists with sickles and sacrifices at Stonehenge. Chain mail just did not fit the image.
DRUIDS: These men are priests of a neutral-type religion, and as such they differ in armor class and hit dice, as well as in movement capability, and are combination clerics/magic-users. Magic-use ranges from 5th through 7th level, while clericism ranges from 7th through 9th level. Druids may change shape three times per day, once each to any reptile, bird and animal respectively, from size as small as a raven to as large as a small bear. They will generally (70%) be accompanied by numbers of barbaric followers(fighters), with a few higher-level leaders (2-5 fighters of 2nd-5th levels) and a body of normal men (20-50).
The prohibition on metal armor doesn't show up until they appear as a class in Eldritch Wizardry. I can see it being inspired by fiction/dubious pop-history, too, but it's at least equally likely to be a pure gamist prohibition, to distinguish them from Clerics and give them different strengths.