Axolotl
Compared with land-dwelling salamanders, the sexually mature adult Axolotl is a chimera (mixture) of larval and adult tissues. A mutation in hormone production slows the development of the non-sexual tissues compared to the gonads. So, it ends up as an adult which looks like a larva, except that it is sexually mature. This is an example of neoteny, a type of heterochrony. Axolotls in the lab have reduced pigmentation and end up being a lighter pink color, while axolotls that live in nature are a darker, brown to black color.