The distinction between historian and poet is not in the one writing prose Aristotle, On Poetics

The distinction between historian and poet is not in the one writing prose and the other verse... the one describes the thing that has been, and the other a kind of thing that might be.  Hence poetry is something more philosophical and of graver import than history, since its statements are of the nature rather of universals, whereas those of history are singulars.  ~Aristotle, On Poetics