Answer:
Male platypuses have a hollow spur behind their rear ankle through which they can deliver a venom powerful enough to kill a dog or cause agonising pain to humans.
People who have been "spurred" by a platypus report that the pain is strong enough to cause vomiting that may last for days, weeks or sometimes even months. The pain cannot be relieved by morphine and other standard pain-killing drugs. It seems the only way it can be relieved is through anaesthesia of the main nerve from the spur site.