Ooh, cryptids are fun! I don’t really believe in any of them, but the most likely one for me would be Nessie (Loch Ness Monster).
I think Nessie being a plesiosaur-type creature someone found the skeleton of is a cool to think about. Nessie looks so similar to plesiosaur that this would make sense, although I don’t know if plesiosaurs occured near Loch Ness.