A legendary cryptid, best known in the vicinity of Point Pleasant, West Virginia. It is described as a large humanoid creature with giant moth-like wings and glaring red eyes. It had a tendency to fly directly at automobiles, badly frightening drivers as it dove into their headlights, then swooped away.
The creature's most active period was just over a year, from mid-November 1966 to mid-December 1967, with its final appearances said to coincide with the mysteriously prophesied collapse of the Silver Bridge, a disaster which claimed the lives of 46 people.
Mothman was probably a sandhill crane, with a heavy dose of public hysteria.
An excellent book on the subject is "The Mothman Prophecies" by John Keel. It should be noted that this is not really nonfiction; many of the facts of the Mothman incidents are accurately depicted, or as accurately as you can get with a book like this. Keel believed in the Mothman 100%, and believed that it and its controllers were from another world -- not really aliens, but what Keel called "ultraterrestrials." Nevertheless, I don't believe most of Keel's conclusions are reliable -- they are what Keel wanted to believe, and there's no actual proof that the Mothman or the sinister Indrid Cold existed, or that they caused the Silver Bridge collapse. Still, this book is noteworthy and essential reading, because it is one of the most terrifying works of semi-nonfiction ever written.