Gross.
Like its previous stage Paras, Parasect is an insectoid creature. However, in this stage the mushroom has completely overtaken the host's body. The insect has been drained of nutrients and is now under the control of the fully grown tochukaso.
Due to the influence of the tochukaso in its growth, the insect's metamorphosis has been stunted and it instead resembles a grotesque, exaggerated version of its former self. Its pincers and legs have grown much larger and the mushrooms on its back have grown together into one giant mushroom. Its eyes have become whited out, likely due to its being possessed by the tochukaso.
These mushrooms that grow on Parasect and Paras are often prized by collectors.