There was a human Pennywise the Dancing Clown, but IT just takes his form. In the movie canon, Pennywise the Dancing Clown was a real person whose image IT decided was a terrifying enough basis for his child-scaring campaign.
There's not really a true story to Art's creation. If there was, it would only serve to make him more terrifying. Terrifier's Art was created when Damien Leone had the idea of a clown annoying a woman on her way home from work on a bus.