Cryptozoology does investigate legendary humanoids, including a number of creatures whose intelligence seems high enough that they probably would not qualify as animals, such as marked hominids. In addition to this argument, there seems to be no reason to absolutely confine cryptozoology to earthly creatures. Although aliens do fall near the fringes of cryptozoology, they do not fall entirely outside its possible sphere of inquiry.

Depending on what explanation you propose for them, aliens could even fall entirely within the rightful realm of cryptozoological inquiry. There are five general explanations that have been used for the alien/UFO fad:
Theory #1: These things are not real (i.e. hoaxes, misidentification or hallucination).
Theory #2: These things are from another planet.
Theory #3: These things are actually advanced technology secretly created by modern governments.
Theory #4: These things are from some other dimension of reality (usually explained as paranormal, sometimes explained with weird physics).
Theory #5: Aliens are actually a species of human, a species that developed high technology ages ago and then retreated to a secluded life in underground or underwater bases or on other planets.

At present, theories #1 and #2 are most popular, but over the entire course of the alien/UFO fad, the popularity of different theories have waxed and waned, with nearly all of them having their day in the sun at some point in time. If aliens are real and theory #5 is true, then aliens would be a sort of converse to Bigfoot. Instead of being more primitive humanoids, they would be more advanced humanoids. People who believe theory #5 tend to lump aliens together with fairies, demons, and angels, using nearly every myth about powerful humanoids to support their claims of an ancient technological race. Sometimes reptoids are also thrown into this mix.

There is a different sort of theory that would also bring aliens more firmly into the cryptozoological fold. Some cryptids are so odd that cryptozoologists have been entirely stumped as to what sort of creatures they might be. Sometimes, researchers theorize that the most bizarre cryptids, such as chupacabras and air rods, are escaped alien pets or genetic experiments. To many people, such a theory would make more sense than the idea that such weird animals have always been here and evolved naturally on Earth. A few people think that nearly every cryptid was seeded here by aliens, or that every cryptid is part of some vast psychological experiment being conducted by aliens.

Along these lines of thought, the most radical ideas on this subject claim that cryptids such as Bigfoot are cleverly-disguised robots of some sort, possibly with biological components, that aliens send to do their bidding. This idea sounds silly to me. If you must disguise your robots as living creatures, why not make them look like real humans or animals that are native to the area? Either you want to hide your robots or you don't.

Some of the above theories linking aliens and cryptids sound nice, but have only the slimmest bits of evidence to support them. One piece of evidence is sightings of hairy dwarfs and other UFO Bigfoot-type creatures. These reports are rare, and seem to be getting rarer. People are seldom claiming that they saw Bigfoot get on a UFO today, and they did not claim it that often in the past either. Still, there are enough well-documented reports of this type to tantalyze some cryptozoologists.

The only other piece of evidence for these theories is suggestive of something, but too unclear to be much use in proving anything. This is the fact that when you make maps of places where cryptids are frequently sighted and places where UFOs are frequently sighted, these places usually match, and often bursts of cryptid sightings and UFO sightings also coincide roughly in time. To some people, this means that cryptids come from UFOs. To other people, it means something else entirely.

You see, these high-cryptid, high-UFO areas (and, to some extent, the times when there are high numbers of sightings) also correspond to those places with the highest levels of ghost sightings and paranormal activity of all sorts. It could be that people from certain areas are more open-minded and less sensitive to ridicule, so that they report all sorts of bizarre things more frequently, from aliens to Bigfoot to ghosts.

As to the times when sightings are high, it may be that, when people are coming forward in large numbers with bizarre tales, it emboldens other people to come forward with other bizarre tales. These matching patterns between UFOs, cryptids and the paranormal are fascinating, but they could be interpreted in so many different ways that it is almost impossible to use them to try to prove anything.

Despite all these theories, you do really have to stretch in order to include aliens among the suitable subjects for cryptozoology. They might be real, and they certainly seem worthy of investigation, but those investigations will probably be left to people other than cryptozoologists.

You can find out more about Aliens from the following sources:

The Alien Zoo

Aliens and UFOs Among Us

Bell, Art. The Source: Journey Through the Unexplained.

Branton. The Dulce Wars: Underground Alien Bases and the Battle for Planet Earth.

Childress, David Hatcher. Extraterrestrial Archaeology, New Revised Edition.

Clark, Jerome. The Ufo Book: Encyclopedia of the Extraterrestrial.

Clark, Jerome. Unexplained!. Detroit: Visible Ink Press, 1999. Pages 46-63, 66-87, 123-127, 142-143, 197-203, 507-511

Coleman, Jerry D. Strange Highways: A Guidebook to American Mysteries & the Unexplained. Alton, Illinois: Whitechapel Productions Press, 2003. Pages 65-72

Coleman, Loren. Mysterious America: The Revised Edition. New York: Paraview Press, 2001. Pages 35-36, 245-248

Corso, Philip. The Day After Roswell.

Cosmic Conspiracies

Cox, Wyatt. Ufo And Bigfoot Sightings In Alabama: A Listing And Examination Of Selected Sightings.

Dolan, Richard M. UFOs and the National Security State: Chronology of a Coverup, 1941-1973.

Eason, Cassandra. A Complete Guide to Faeries & Magical Beings. Boston: Weiser, 2002. Pages 10-11

Evans, Chris B. Alien Conspiracy: Unraveling the UFO/Alien Mystery.

Glenday, Craig. The Ufo Investigator's Handbook: The Practical Guide to Researching, Identifying, and Documenting Unexplained Sightings.

Hopkins, Bud.Sight Unseen : Science, UFO Invisibility and Transgenic Beings.

Hynek, J. Allen. Night Siege: The Hudson Valley Ufo Sightings.

~Authors~ Would you like your nonfiction book indexed in The Cryptid Zoo? Ask if you can send a review copy.

Hynek, J. Allen. The Ufo Experience: A Scientific Inquiry.

Keel, John A. The Complete Guide to Mysterious Beings. New York: Doubleday, 1994. Pages 65-66, 68, 81, 99-100, 102-104, 124, 127, 141, 146-211, 319-326

Kossy, Donna. Strange Creations: Aberrant Ideas of Human Origins from Ancient Astronauts to Aquatic Apes.

Marrs, Jim. Alien Agenda: Investigating the Extraterrestrial Presence Among Us.

Newton, Michael. Encyclopedia of Cryptozoology: A Global Guide to Hidden Animals and Their Pursuers. Jefferson, North Carolina: McFarland & Company, 2005. Page 436

Out of the Blue - The Definitive Investigation of the UFO Phenomenon

Rath, Jay. The W-Files: True Reports of Wisconsin's Unexplained Phenomena. Black Earth, Wisconsin: Trails Books, 1997. Pages 43-106

Sanderson, Ivan T. Invisible Residents: The Reality of Underwater UFOs.

Sitchin, Zecharia. The 12th Planet: The Earth Chronicles, Book 1.

Steiger, Brad. Out of the Dark: The Complete Guide to Beings from Beyond. New York: Kensington Books, 2001. Pages 241-266

UFOs & Aliens

Wikipedia, The. Ancient Astronaut Theory

Wikipedia, The. Extraterrestrial Life

Wikipedia, The. UFO

Wikipedia, The. Ufology

Wikipedia, The. UFO Religion