If you ever find yourself in central Africa, here's a piece of advice: don't mess with the frogs. As Ed Yong over at "Not Exactly Rocket Science" warns us, several species of them come equipped with a set of bony Wolverine-like claws that punch through their skin when threatened.
Nature's full of clawed animals, but the frogs' defense mechanism is unique in the natural world because their claws literally rip through the skin when extended. They're also made of bone instead of keratin (sorry, no adamantium claws are known to exist in reality, except for this guy). Researchers aren't sure if the claws are retractable or not, and as Yong notes, they may never really want to find out:
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.
In the X-Men movie, Wolverine, when asked if it hurts to pops his claws, answers, "Every time." One can't help but think that the same is true for the frogs.
However, many amphibians have extraordinary healing abilities that can even regenerate severed limbs. It may be that the clawed frogs, like their comic-book counterpart, have a 'healing factor' that closes up the wounds that open every time their claws are used.
Source: Not Exactly Rocket Science