Haiku, Sonnet, and Opus also made sense to me, but Fable ruined it. It isn't at all clear that a fable is bigger/grander than someone's magnum opus—actually, I'd think probably the opposite.

I don't think it gets better if you swap "fable" for "myth" either, although when you add "-os" to the end it suddenly sounds grander so there's that. Mythos isn't a model I can use, though.