We all know what a dragon is. We see countless pictures of this beast on the covers of fantasy novels. The typical artwork shows an animal with a body that roughly resembles something between an exotic lizard and a dinosaur, with scales, claws, four legs and often leathery wings. The winged varieties thus generally have six limbs. The body is typically thicker than the tail, not thin and serpentine like Asian dragons. The head is vaguely horse-shaped, with large eyes and a long snout. Often, this creature breathes fire.
What you might not know is that there are actually reports of dragon sightings in Europe and America. Often the stories are told as first-hand, and they date from all eras of history, even some from quite recently. For example, there is the 92-foot flying dragon-like creature that was reportedly killed by Arizona ranchers in 1890.
The dragons described in legends from North America and Europe are similar enough to fantasy-novel dragons that we can still recognize them as dragons, but there are some substantial differences. For one thing, there are dozens of different varieties. Folkloric dragons generally have four legs and two wings. The smaller wyvern has only two legs and two wings. Lazy modern film makers often pass wyverns off as dragons. The true dragon with six limbs (two wings, four legs) is depicted as far back as ancient Babylon.
A few of these beasts are landbound and wingless. Some of these landbound dragons have four legs, while others have only two, generally located towards the head end. The English "worms" are not only landbound, but are rather snake-like and resemble the classic dragons of Asia. There are many reports of European dragons which seem like sea serpents that have crawled up onto the land, sometimes complete with paddle-legs.
Classic European dragons from folklore frequently resemble huge crocodiles or huge snakes in some way, and it is also quite common for them to have one or more characteristics resembling some non-reptilian animal, such as a sheep's head. Classic European dragons often have supernatural powers and are sometimes thought to be demons or minions of the devil. They often breathe fire and/or exude poison (especially having poisonous blood, but sometimes also giving off poisonous fumes from the mouth or from the body in general). Folklore dragons are often thought to be responsible for extreme weather events, causing hail, lightning and intense storms.
North America and South America also have their dragons, which often resemble the classic European varieties more than the classic Asian varieties. For example, there is the gowrow of Ozark lore from Missouri, a twenty-foot-long reptile with big tusks, and the piasa, a scaled flying beast with horns.
The name in cryptozoology that is currently most associated with the search for dragons is probably Richard Freeman, author of a Dragons: More than a Myth, a book published by the Centre for Fortean Zoology. Very few scientists stand with him. Modern cryptozoologists are not very interested in dragons. For one thing, even though modern reports exist, these reports seem to be much less common than they were in the old days. In addition, there is a big problem with organization and terminology. It is clear that dragon legends, even if you limit yourself to western cultures, describe at least a dozen different creatures. Therefore, the proper scientific method demands that we separate the different kinds and study each variety by itself. The book itself however points out that dragons are not one type of creature but a rich tapestry of legend woven from encounters with a number of different reptilian beasts.
This means that cryptozoologists end up studying dragons under different names. Some reports are classified as possible surviving dinosaurs, some as giant crocodiles or big snakes, some as sea serpents who are briefly traveling over land, and some as pterosaurs or possible giant bats.
In 1979 author Peter Dickenson wrote a book of speculative zoology called 'The Flight of Dragons'. In it he theorized that dragons evolved from large, flesh eating dinosaurs like Tyrannosaurus. They flew and breathed fire by manipulation of hydrogen gas.