Not only part of a Shakespearean insult, a Shambling Mound can also be a creature out of D&D, too.
I will not go into too much statistical detail (for copyright reasons), but here are some basic facts about this big, muddy, and smelly thing
Shambling mounds dwell in thickly over grown swamps or in exceedingly wet and fetid subterranean locals. Mounds will feed on anything organic, including other plants and animals, as well as the living and the dead. Other questions, such as how they breed and how long they live, is unknown. Some believe that shambling mounds are enraged manifestations of the swamps in which they dwell. Others say that are elemental spirits gone wrong.
A shambling mound is a tall heap of rotting vegetation. It smells rank and water runs freely from the shambling mound's body. The creature also drips slime and liquid mud as it walks.
As a personal note, Shambling Mounds are one of the AD&D creatures that really make you wonder how you're supposed to kill it. Weapons don't really harm it. Fire doesn't harm it. Cold does reduced damage. Lightning makes it get bigger... I guess you could disintegrate it... Anyway, a shambling mound actually looks like it could be fairly useful in a supers game (instant swamp monster), a fantasy game, a Shadowrunish game (toxic swamp spirit?), or a pulp horror game (like a minion of the Elder Gods).
Even more creepy, while the Shambling Mound may look humanoid, its vital area (or "brain") lies near the center of its body, so foolish PCs often try to chop its head off and, well, waste their time and energy on it.
I have no idea how you get rid of it (a lot of big guns?) and I sure don't envy the party of PCs that meets one.