A plant that moves, and can attack others. Usually, this involves a plant-like form of Tentacles using vines. Other than this, or in conjunction with this, they may act like a pitcher plant, venus fly trap, or other predatory real-world plant, dragging characters into a deadly portion of the plant.
Plant monsters probably shouldn't be humanoid. Plant Girl is used for (feminine) humanoid plant monsters, while Plant Boy is used for (masculine) humanoid plant monsters.