Crested geckos are a species of gecko found in New Caledonia. More information can be found here: . Crested geckos are believed to be a inter-universal constant, due to their remarkable ability to exist in all known dimensions and on at least one known planet with the right conditions for crested geckos in those dimensions.
A common theory is that all universes contain at least one planet with crested geckos on it but often containing more. It is believed that the species will evolve but a group will remain unchanged to continue the crested geckos. It is also believed that if crested geckos become entirely extinct in a universe that universe will become 'doomed' and will perish within a few thousand years of them becoming extinct.