George M. Eberhart's Classification for Cryptozoological Creatures

1. Distribution anomalies - animals in places they shouldn't be; e.g. anomalous big cats of the UK
2. Undescribed, unusual, or outsized variations of known species - e.g. giant anaconda (Amazon); spotted lions (East Africa)
3. Survivals of recently extinct species - e.g. ivory billed woodpecker, presumed to be extinct in 1960; Stellar's sea cow, presumed to be extinct in 1770; both of which are occasionally claimed to have survived into the present
4. Survivals of species known from the fossil records into modern times - e.g. mokele-mbembe (Central Africa) is sometimes described as a living dinosaur
5. Lingerlings, or survivals or species known from the fossil record much later into historical times than currently thought - e.g. wooly mammoth, presumed to be extinct in 12,000 BCE, but is occasionally purported surviving into later eras
6. Animals not known from the fossil record but related to known species - e.g. Andean wolf; striped manta ray
7. Animals not known from the fossil record nor related to any known species - e.g. Bigfoot; sea serpents
8. mythological animals with a zoological basis - e.g. the griffin, partly inspired by dinosaur fossils of Central Asia
9. seemingly paranormal or supernatural entities with some animal-like characteristics - e.g. Lizard Man; Mothman; Black Dogs; faeries
10. Known hoaxes or probable misidentifications - e.g. jackalope, possibly inspired by rabbits infected with Shope papilloma virus, which causes antler-like tumors

Exclusions:

1. Insignificance - cryptids must be big, weird, dangerous, or significant to humans in some way.
2. Lack of controversy - someone needs to observe a mystery animal and someone else needs to discredit the sighting; cryptozoologists function as interventionists between the witnesses and skeptical scientists
3. Erratics - the out-of-place alligator...that turns up in an odd spot, undoubtably through human agency, is not a zoological mystery...[if] someone discovers a new species of alligator that lives only in sewers, that is a different matter
4. Bizarre humans - e.g. zombies
5. Angels or demons - ...the paranormal is admitted only if it has an animal shape (a werewolf sighting, which might involve a real dog, a wolf, or a mystery canid
6. Aliens - [unless such extraterrestrials] arrived a long time ago and thus classify as residents