"Some myths describe some actual historical event, but have been embellished and refashioned by various story tellers over time so that it is impossible to tell what really happened." Encyclopedia Mythica.
A mythomaniac is someone who embellishes and refashions actual events consistently about most every aspect of their life, even seemingly trivial events. There's a level of emotional charge that becomes easy to spot as the embellishing and refashioning happens. It's as if the mythomaniac has learned that making the statements bolder with great enthusiasm ensures that the listener will believe it without checking the facts.
The mythomaniac will exaggerate his or her education, accomplishments, skills, value of possessions, community contributions and relationships. They are huge when they need to be huge. They are small when they need to be small. Those who have known the mythomaniac over time, know it's important to use caution in buying into any story the person tells. If the mythomaniac is aware of that, it doesn't always show. But they will try to isolate anyone who may expose myths. And they will be very nasty in creating myths in attempt to ensure that isolation takes hold.
Decent and polite people have a difficult time dealing with a mythomaniac. It's hard to believe someone would create total fictions and when there are seeds of truth within, it's not easy to sort out. It takes energy. It wastes energy. So, the mythomaniac goes on without confrontation.
Synonyms for mythomaniac would be con artist, pathological liar.
Do you have a mythomaniac in your life? I hope not because I know how difficult they can be to deal with. But know this - they cannot keep track of all the myths they tell. Time exposes them.