Where does the shroom on the head come from?
The parasitic mushroom comes from real life. Cordyceps



From http://i.guim.co.uk/static/w-620/h--/q-95/sys-images/Guardian/Pix/pictures/2012/5/3/1336047375506/Zombie-ant-infected-with--001.jpg

This fungus infects insect and uses them to breed. This is where Game Freak got its inspiration for Parasect.