The frogs defend themselves with sharp bone claws on their hind feet but to do so, the animals have to drive the claws through their own skin. It's an extreme defence that is completely unique in the animal world.
The clawed frogs belong to a family called Arthroleptidae that were discovered in Central Africa more than a century ago. At first, people wondered if the claws just stuck through the skin as a side effect of the preservation process. Alternatively, the frogs may have used them to grip or climb. Their true function as defensive weapons only became clear when naturalists first described actually picking up and handling live animals.
Doing so is a mistake, and anyone who makes it is punished with a series of deep, bleeding wounds inflicted by the struggling animal as it kicks out violently with its claws. The ability is well known to the people of Cameroon, who only ever hunt the frogs with machetes or spears.
Scientists are still not sure what happens to surviving frogs after the claws are no longer needed. Link