Poster, I’m not sure what happened to your description, but you might want to add some more details about what you’re looking for here in the comments.
Quest for Camelot (1998)
Might be Rankin/Bass’ 80s film “Flight of Dragons.” I dunno why, it was the first thing that popped into my head. It was animated, and the late great John Ritter voiced the human protagonist who (as I recall) becomes a dragon for most of the film.
Pete’s Dragon (1977) with a cartoon Dragon?