https://en.m.wikipedia.org/wiki/Hairy_frog

The hairy frog is also notable in possessing retractable “claws” (though unlike true claws, they are made of bone, not keratin), which it may project through the skin, apparently by intentionally breaking the bones of the toe.^[1] In addition, the researchers found a small bony nodule nestled in the tissue just beyond the frog’s fingertip. When sheathed, each claw is anchored to the nodule with tough strands of collagen, but, as Gerald Durrell^[4] discovered firsthand, when the frog is grabbed or attacked, the frog breaks the nodule connection and forces its sharpened bones through the skin.