Theories about Nessie Existence

Many families of faery monsters are supposed to live in lakes and under the sea.  There was a widespread Highland belief that a water horse, kelpie or each-wisge, lived in almost every lake in Scotland. Not to forget the tarh-uisge, a sea bull that sometimes fornicated the local cows.

These evil spirits were believed to take pleasure in luring travellers to their drowning deaths.
Possibly this myth stemmed from a need to explain mysterious drowning accidents or to lure away children from the slipping banks of rivers and lakes