Often referred together with Monstrous Fungus.
Monstrous Plants are generally carnivorous plants that have grown to large sizes either by magic or plain unstopped growth much like a tree is theoretically able to grow forever given the conditions. Identified non-carnivorous monstrous plants are few in number, presumably due to the required amount of nutrients and much lower risk of harm (at most being poisonous as a means of protection from being eaten) that makes such classification gray compared to plain wide-spread infestation of a single plant.
While rare to find in places with humanoid populations who are able to quickly cut down on a given plants spread, such plants are massive threats in jungles and uninhabited forests where they are able to spread unopposed.
Monstrous plants are generally mindless much like constructs or most undead, only going off basic response stimuli, such as warmth, specific smell, or physical touch.