Another one for when/if druids come in (as there will be plant affecting spells)
Treants: Large intelligent mobile tree
Tendriciculos: Huge carnivorous plant beast.
Vegepygmy: Humanoid Plants
Assassin Vine: Animated poisonous vines
Shambling mound: Hard to kill plant golem type thing
Myconid: An underground dwelling humanoid psychic fungus but still could come under plant type