Monsters are freaks of nature or scary creatures in stories that are ugly and mean. Technically, something like a two-headed snake would be a monster. While a two-headed smake is possible physiologically, most people think of monsters as creatures with diverse features that don't biologically co-exist.
While some "monsters" are believed to be separate species, such at Yeti and Bigfoot, Chupacabra, and Dragons. Others are truely unique, such as the Lochness Monster, Mapinguari, Mothman and The Jersey Devil. Mosters are usually products of legend. Perhaps rooted in something factual.
The Jersey Devil is a legendary creature or cryptid said to inhabit the Pine Barrens of southern New Jersey. The Jersey Devil is the subject of a legend dating from the early 18th century. There are several variations, but a common story holds that a woman named Mother Leeds gave birth to her 13th child on a dark and stormy night. Rumors claimed that she was a witch, and bore the Devil's child. Shortly after birth, it changed form, growing wings, hooves and an equine head. It flew into the air with a bloodcurdling shriek, killing a midwife in the process, and headed toward the woods. The creature is often described as a flying biped with hooves, but there are many different variations.