In its simplest explanation cryptozoology is the X-Files of the zoology world. Literally breaking the word down it is derived from the Greek kriptos meaning hidden and zoology meaning the study of animals. So in this literal interpretation it is the study of hidden animals, or animals that are largely considered to be legendary or extinct. This study can range from proving certain dinosaurs are not really extinct to searching for the legendary Bigfoot or the Chupacabra. The only requirement for an animal to be studied by cryptozoologists is that the physical evidence of their existence is scarce yet the myths and reports of them remain. It is also worth noting that many scientists view cryptozoology as a pseudoscience. Papers on the subject are hardly ever published and no formal training exists for the science. No known scientist has ever been hired specifically for the research of cryptozoology.
The term cryptozoology is often attributed to zoologist Bernard Heuvelmans, though he attributes the term to Scottish explorer Ivan T. Sanderson. In his books on the search for legendary creatures Heuvelmans argued that with so much unknown about the natural world cryptozoology should be undertaken with fervor and rigorous discipline, but an open mind as well. He stressed the importance of local folklore and urban legend as sources for discovering these fantastic creatures. He argued that while many of these tales are layered with unlikely and impossible elements, there is a grain of truth in them all.
Willy Ley is also an important name in the world of cryptozoology. While he is best known for writing about rocketry, his training in paleontology led him to write the book Exotic Zoology (1959). In the book he mainly discusses the possibility for the Yeti, sea serpents and some species of dinosaurs' continued existence. Yet he also goes on to hypothesize that legendary creatures like the unicorn or cyclops might have actually existed in the ancient world. British zoologist and cryptozoologist Karl Shuker is considered an expert in the field, having published 12 books on the subjects since the mid-80s.
Many are critical of cryptozoology since it relies heavily on anecdotal evidence and stories. Typically the scientific method is not followed and much time is spent pursuing animals that most evidences suggests they likely do not exist. While most scientists won't dispute that there are thousands of unknown species of animal out there they criticize the cryptozoologist for being uninterested in researching new species of insects or reptiles and instead pursuing elusive creatures that only exist in legend. These same critics suggest that searching for Bigfoot or the Loch Ness Monster is a waste of time since their size would make it unlikely for them to exist on a large enough scale to have a breeding population yet remain undetected.
Supporters of cryptozoology point out that large animals of unknown species are being discovered all the time, making it likely that legendary creatures could prove to be just as elusive as well. They use this claim to defend that their research is valid and that criticism is mainly based on a popular world view rather than the merit or failure of the research itself. Cryptozoologists also contend that they are largely responsible for the most compelling evidence that the object of their search does not exist. Some of the best evidence disproving the existence of Bigfoot, for example, came from cryptozoologists. Similarly, they claim many animals now accepted by zoologists were once thought to be myth. The Mountain Gorilla is one such creature. It was thought to be a hoax or a myth but is now accepted as a species by the general scientific world.
While it can't be argued that there are many mysterious left to the world of zoology, even cryptozoologists suggest being cautious. There may be a discovery of a new species tomorrow, but this should not be used to too broadly excuse the search for creatures that most evidence suggests is merely myth. Even cryptozoologist Ben S. Roesch acknowledges that the discovery of a new species shouldn't lead anyone on a blind hunt for a creature that there is no evidence of its existence or that has largely been disproved.