|The axolotl (Ambystoma mexicanum)|
|An albino axolotl|
This animals exhibits what is called neoteny, which is when a creature remains in a body form that is usually considered juvenile in nature. Most amphibians have external gills and a "keeled" fin-like tail when the are born, both of which will be lost when they undergo metamorphosis and become adults. Axolotls, along with several other species of amphibian, actually keep the gills and keeled tail into adulthood. This usually happens in animals that remain aquatic in nature. The external gills are most noticeable in the albino individuals of this species because of the bright red color against the pale skin. Compare the two images I provided and you'll get what I mean. Oddly enough, on rare cases they will undergo metamorphosis anyway, in which case they end up looking remarkably like the better-known tiger salamander.
A native of Mexico, the axolotl is, unfortunately, an endangered species. However, because there are so many in captivity it is unlikely the species will ever die out completely, even though it is quite possible that it could be lost from the wild.
Below is a video on regeneration in this species:
Images are from Wikimedia Commons under creative commons licenses: one, two