We have robots, human, animals, mutated animals and animal/robot (cyborg) mobs.
Perhaps we can also have plant based mobs?
Not necessarily mobile mobs - they could be static projectile throwers, or booby trapped plants (attack when you touch them).
Plants could also trap players (e.g. a Venus fly-trap like creature) and attack the player until they have to have their rest.
Plants could also be immune to damage by bullets (anyone killed a tree in RL by shooting one to death?), damageable by impact type weapons (sword, axe, bat).
A google of plant based monsters should give your artists some categories to design.