This exact concept is from Plato's Republic and is typically called the "Noble Lie" in the philosophical literature (it seems clear from the Republic that Plato invented the myth, it wasn't actually one previously in currency among the ancient Greeks). It's not clear whether Plato himself considered it a lie rather than just a "creative" way of representing a deeper Truth.
The term itself is still in use, but with a narrower scope than what you are seeking, chiefly meaning a fiction promoted as truth to maintain social harmony, with altruistic intentions.
A similar phrase with wider scope (if lesser usage) is "useful fiction," meaning something known to be untrue, but of pragmatic use. I wasn't able to find a definitive citation for this definition, but the term does have some usage, both within and outside philosophy, and is easily understood (there is even a book on the subject).