This salamander can regrow an entire limb, jaw, or piece of its own brain

Axolotls can lose a leg, tail or even sections of heart and brain tissue and simply regrow them — complete with bone, muscle, nerve and skin, and without scarring. Unlike most animals capable of some regeneration, axolotls do this as fully mature adults, which is why regenerative-medicine researchers study them closely for clues about why humans lost this ability somewhere in our evolutionary past.

— Susan V. Bryant & David M. Gardiner, Retinoic acid, local cell-cell interactions, and pattern formation in vertebrate limbs — Developmental Biology, 1992