Axolotl

Like their frog and toad cousins, salamanders begin their lives in a water-breathing larval stage. These salamander "tadpoles" resemble stunted adults with flatter tails and feathery, external gills. There's a few species, however, that spend their entire lives in this stage...at least, they're supposed to. Axolotls are large, hardy, cold-water salamanders that normally retain their gills for life, but under specific environmental conditions they can metamorphose into an "adult" form that rarely lives longer than a few weeks. This unusual process is a throwback to their evolutionary heritage, and usually occurs only under human intervention.

Axolotls are native to just one lake in the world, but boast a massive captive population as aquarium pets, and are often also studied for their exceptional regenerative capabilities.