Nessie is not a necessary example, but shouldn't be readily thrown out either. 11,000 witnesses (including a member of parliament) can't all be kooks. Also, I know that the BBC recently trolled Loch Ness with sonar equipment, but their research does not prove that Nessie does not exist. Loch Ness was given a thorough workover by, I believe, the Japanese several years ago and they note that the bottom of it is full of canyons and large caves. With a bunch of boats going overhead, it's possible the creature was hiding.
Nessie-type animals have been seen on almost every continent. Champ is one such creature (could also be a zeugolodon (sp?) or some yet unidentified species. Ogopogo is another. While these "sightings" do not 100% confirm that the plesiosaur still exists, the evidence is intriguing.
This site has a lot of information (not all supportive of the plesiosaur theory, but not all discounting either) regarding the existence of such creatures. Remember, names like "dinosaur" are of recent origin.