The Plesiosaurus

The plesiosaurus is a prehistoric dinosaur that lived in the cretaceous period and that looks just like Nessie, or what the people are describing Nessie to look like. It grew to be twenty three meters long, making it among some of the largest sea creatures. It was a very successful reptile that thrived during the age of the dinosaurs. The plesiosaur was easily identifiable by its four large flippers. The neck of this animal was very long.  The plesiosaurus lived and thrived up until about 65 million years ago. Many plesiosaur fossils have been found along the coast of England. Many people have theorized that the modern day loch ness monster, along with many other myths are modern day plesiosaurs. So, people might actually be seeing a plesiosaur rather than Nessie.
Information from http://www.dinosaurfacts.org/plesiosaurus
Shown below is a picture of a Plesiousaurus. Very similar to Internet pictures of Nessie.