I’ve put vegepygmies in my campaign in one of the early sessions (here) and two things stood out at the time: they’re creepy as hell AND much more effective than a quick look at their statblock might suggest (even in 5E). Furthermore, with one quick change we could add a lot of variety and quite a few wrinkles to them. But first, a few words about their origins.

Vegepygmies first appeared in Expedition to Barrier Peaks (Gary Gigax, 1980), a science fantasy adventure in which a surge of strange monsters is explained by the presence of a crashed spaceship in the vicinity, with its weird released denizens.

Here’s part of the monster block for vegepygmies in EtBP:

Vegepygmies are vegetable creatures of low intelligence. They organize themselves in regional bands, living by scavenging and hunting. Meat form their diet and they will eat it no matter what its condition.

Attacks from piercing weapons such as arrows and spears do only 1 point of damage to vegepygmies. Electrical attacks do no damage. Cold and fire do normal damage. Vegepygmies are immune to all charm/enchantment spells except those that affect plants.

They co-exist well with other forms of plant-life.

Vegepygmies are short bipedal creatures with sharp thorn-like claws. Their shoulders, abdomens and limbs are fringed with leaf-like tendrils. Their heads have a topknot of small leaves. They eat carrion and meat. Vegepygmies reproduce by russet mold or by propagating buds from their bodies.

Also:

“Any character who dies from contacting russet mold will undergo
a transformation. First, their body will immediately start to sprout
new growths of mold from the spores left at contact. Then, when
entirely encased in mold, a vegepygmy (cf.) will rise from the
remains. “

So vegepygmies are plant-people but are closely associated with mold (which Gigax seems to confuse with plant life). Anyway, their next appearance, in the Monster Manual, simplifies the vegepygmies as being “Mold Men”. And I tend to agree with this.

My own “naturalist” take on the vegepygmies, in my Ruins of Chult campaign, is that they are in fact indigenous batiri goblins that have entered a symbiotic state with a mold life form a long time ago. Thus, vegepygmies cannot infect other humanoids (that’s pretty much why they’re always small).

Diversity! (of mold)

Now, an interesting addition I think is that there could be many species of infecting molds and not just the aforementionned russet mold. Brown Mold, considered a hazard in D&D, is an interesting example. Specifically designed for those annoying players that always burn down any problem they encounter (I know mine do), brown mold THRIVES in fire (but hates cold). Replace russet mold by brown mold and you’ve got fire-loving vegepygmies!

Or really, we can just invent any whatever-mold and ascribe unusual features to these lovable Mold Men!