People have always been fascinated with the hidden, the mysterious, and the unexplained. Every society has its tall tales and ghost stories, its odd legends, and heroes. Also, every society has its stories of strange beasts, dangerous or benign, that live in the twilight world between the everyday and the legendary. Through most of history, people have been closely tied to nature, hunting in forests and having an intimate knowledge of the animals in their regions. So-called "primitive" peoples were walking encyclopedias of the natural world, and yet most believed there were more creatures lurking in those woods than the ones they usually encountered. Even as the world becomes more connected, the belief in strange creatures continues as strong as ever.

Indeed, the willingness to believe in exotic animals has been so widespread that some have made careers out of displaying "oddities" to the public at circuses, fairs, and museums. Perhaps the most notorious individual to do this is P.T. Barnum, whose New York City museum was so popular in part because he was more than happy to invent items with which to fascinate the public, even if no such item actually existed. His first example of this was the now famous "Fiji mermaid." Barnum rented this oddity from a Boston rival, Moses Kimball, in 1842, but while the creature floating in the jar of formaldehyde was described as a mermaid, it was actually the body of a very young monkey with a fish tail sewn on over its legs. Barnum leased the item long term for $12.50 per week and then marketed it as having been caught by his friend Dr. J. Griffin, a pseudonym for Barnum's business associate Levi Lyman. For his part, Barnum saw nothing wrong with what he was doing, justifying the hoaxes by saying they were just "advertisements to draw attention...to the Museum...I don't believe in duping the public, but I believe in first attracting and then pleasing them."

Although cryptozoology is often scoffed at and widely considered a pseudoscience, one of the reasons it made men like Barnum rich and continues to fascinate people today is the fact that people realize they've only scratched the surface when it comes to identifying all the different forms of life on Earth. As Martin DelRio pointed out in The Loch Ness Monster, "Animals previously unknown to science have been found more than once in the past hundred years. For instance, there's the megamouth shark (megachasma pelagios), a fifteen-foot-long creature weighing nearly a ton. The first specimen was discovered on November 15, 1976, when it was found entangled in the drag anchor of a U.S. Navy ship. The new creature wasn't described scientifically until 1983...The megamouth remains the only species in its genus, and the only genus in its order."

As a result, in one sense it shouldn't be surprising that people across the United States have reported seeing weird cryptids, including giant birds, sea serpents, wolfmen, and a creature so bizarre it defies any rational classification. The only thing these monsters have in common is that all have been seen regularly and reported by seemingly sober, rational witnesses in the United States, even as conventional science suggests these monsters cannot exist and that all the sightings are mistakes, misidentifications, or hoaxes.

For example, a very strange creature had a series of encounters with several witnesses in 1952 in West Virginia that may have been associated with a UFO, and it eventually became known as the Flatwoods Monster. Others have claimed to see a humanoid frog, a flying creature that seemed to be associated with an accident that killed almost 50 people, and one of the most famous folk legends across the Americas: the chupacabra.

Some of the sightings cannot be explained away easily, but at the same time, none of the creatures fit into a rational, conventional view of the world.