Platypus

The bizarre platypus, once thought to be a hoax and now famous as one of the few mammals that lays eggs, is also unusual when it comes to defensive methods. The male platypus has a sharp, retractable spur in each hind limb, connected to a venom gland. When grabbed by a predator (or by a curious or ill-informed human), the platypus kicks with these spurs injecting the venom, which is usually enough for the enemy to let it go. Although the venom can kill animals up to dog-size, it is not lethal to humans. However, people who have been “stung” by these animals claim that it is among the most excruciatingly painful things that can be experienced, and the effects of the envenomation can last for days and temporarily impair the victim. Some claim that the pain is strong enough to cause the victim to faint.

Interestingly, only the male platypus has a functional venom spur; the female poses no threat to other animals (except for the small invertebrates that make up its food, of course). This suggests that the venom spur is primarily an intraspecific weapon, used by male platypus against each other during mating season duels.