Axolotls (Ambystoma mexicanum) are among the weirdest of amphibians. They have amazing regenerative abilities, breed readily in captivity, and have large embryos (good for observing the development of vertebrates). What they are most well-known for, however, is their habit of never growing up.
Well, O.K., that's not completely true. They do become sexually mature, but they never lose the look of their baby stage. Axolotls do not undergo metamorphosis. Yes, they can be coaxed into an adult form (which looks very similar to a tiger salamander), but it does not live long. Instead, axolotls look like bigger babies as they get older, retaining their gills, paddle-shaped tails, and other traits that most salamanders only have as larvae.
Like zebrafish, they can also be genetically-engineered to glow in the dark. You know I'm down with that.
Unfortunately, despite their prevalence in captivity, axolotls are critically endangered. They are only found in two lakes in all of Mexico. One of these lakes was drained because of flooding; the other is a puddle of its former self. The Aztecs also ate these creatures, leading to their use in modern Mexican cuisine, but I could NOT eat this face: