Nymphs of Poisonous Plants were evil spirits, specifically nymphs, who served the White Witch. They participated in Aslan's sacrifice at the Stone Table and likely fought in the First Battle of Beruna. It is unknown whether the plants became poisonous when the nymphs turned bad or whether all poisonous plants were evil.
Most, if not all, of these nymphs were wiped out by the High King Peter and his brother and sisters shortly after the war.