Shambling Mound
The Shambling Mound, or Shambler, is a plant monster from Dungeons & Dragons. First debuting in The Strategic Review #3, August 1975, it has become an iconic creature, appearing the baseline Monster Manual for every single edition of D&D from Advanced Dungeons & Dragons 1st edition to Dungeons & Dragons 5th Edition.
Shambling Mounds are hulking, roughly humanoid masses of vegetation and swampy muck, described as either partially decaying or as a verdant mass of living greenery, depending on the edition. Mindless predators, they roam swamps and jungles, moving with surprising stealth for such large creatures, attacking living beings and killing them with either brutal bludgeoning blows or by smothering them under its bulk; it then consumes the decomposing flesh for nutrition. They have a strange affinity for lightning; they are usually immune to it, and sometimes are actually empowered by it, similarly to a Flesh Golem.
Depending on who you ask, Shambling Mounds were inspired by either Marvel Comics' Man-Thing, DC Comics' Swamp Thing, or even just The Heap, a Golden Age monster-hero character from Hillman Comics who basically inspired both of the latter.
The D&D Cartoon episode "The Prison Without Walls" revolves around the child adventurers having to rescue a gnome wizard who has been polymorphed into what is clearly a Shambling Mound by the evil mage Venger.
An "Ecology of the Shambling Mound" run by the Monster Hunter Association appeared in Dragon Magazine Annual #2, their third chronological appearance.