Axolotls are most well-known for their remarkable ability to re-grow amputated limbs. Their regenerative abilities come with eternal youth - that can be ended with a bit of iodine.
Axolotls are small amphibians, that can be found - with increasing difficulty - in lakes in Mexico. Their name comes from the Aztec language of Nahuatl, and is best translated as 'water-dog.' The little creatures spend their time underwater. They didn't always. They are examples of extreme neoteny.
Neoteny is the retention of juvenile physical characteristics. It can be seen in the puppyish characteristics of some adult species of dogs, and in the human ability to drink milk even as an adult. Lactose intolerance was one way, in mammals, of getting older siblings to stop nursing once younger ones came along. Sooner or later, animals are biologically urged to put away childish things. Axolotls never do.
These little creatures stay in juvenile form their entire lives. At some point, their species diverged from tiger salamanders, which look like axolotls as juveniles, but mature in order to reproduce. Axolotls skipped the maturation, and somehow managed to reproduce anyway. They grow larger than juvenile tiger salamanders, but otherwise stay as they are.
That is, unless people decide to get involved. Scientists discovered that, when an axolotl was given a shot of iodine, it experienced a rush of hormones that caused it to suddenly 'grow up' to what looks like tiger salamander. So although the axolotl lives a separate life and reproduces only with its own kind, it can 'grow into' a different species.
Axolotls are popular pets, possibly in part because their owners wield the power of a god over them, but giving one a shot of iodine outside the lab is a bad idea. The iodine when not precisely dosed, will kill the axolotl outright. Even if it does not, artificially 'matured' axolotls are generally feeble and listless, only living for about another year. Still, it's a fascinating transformation — and one which shows the quirks of evolution.