"Indian legend has it that the large lake creature, Ogopogo, was originally a demon possessed man who had murdered a well known and respected local man named "Old Kan-He-Kan." In memory of this man, his people named Our beautiful lake "Okanagan." To pay for his sins, the Indian gods changed the murderer into a lake serpent so he would forever be at the scene of his crime and suffer eternal remorse. The creature's name became "N'ha-A-Itk" which roughly translates into sacred creature of the water, water god or lake demon." (http://sunnyokanagan.com/ogopogo/)
** REMEMBER... The bottom line is to clearly address the 5 main areas: 1. Background 2. Unreasonable theories 3. Reasonable theories 4. What still remains a mystery 5. YOUR own theory ***You are not just describing your mystery, there should be a purpose to everything that you are telling the audience. It should fit into one of those 5 areas listed above - not just eyewitness accounts (those might help to prove the theory though). You can do it!!