TIL that axolotl are neotenic. There are rare instancea of axolotl going through metamorphosis to become terrestrial. However, if this occurs after sexual maturity the axolotl will not survive more than one or two years.
 "axolotl ... is a neotenic salamander related to the tiger salamander." Wikipedia
Another neotenic salamander species is the Olm, which is an old world species that is blind and lives in caves.