This haiku from Clark Ashton Smith (CAS) was unpublished in his lifetime, and is not available on The Eldritch Dark, so here's the complete text:
In my mule's long shadow
On the moon-wan waste,
Hung Empusa, horror-faced.
One of the original sources for the bizarre creature known as the Empusa is Aristophanes' comic play Frogs, the story of the god Dionysus' journey to the underworld to retrieve the dead playwright Euripides. Dionysus makes the journey with his slave Xanthias, and upon alighting on the shores of Pluto's kingdom, the duo encounter this most curious of monsters:
XANTHIAS: Hallo! I hear a noise.DIONYSUS: Where? what?XANTHIAS: Behind us, there.DIONYSUS: Get you behind.XANTHIAS: No, it's in front.DIONYSUS: Get you in front directly.XANTHIAS: And now I see the most ferocious monster.DIONYSUS: O, what's it like?XANTHIAS: Like everything by turns. Now it's a bull: now it's a mule: and now the loveliest girl.DIONYSUS: O, where? I'll go and meet her.XANTHIAS: It's ceased to be a girl: it's a dog now.DIONYSUS: It is Empusa!XANTHIAS: Well, its face is all ablaze with fire.DIONYSUS: Has it a copper leg?XANTHIAS: A copper leg? yes, one; and one of cow dung.
In CAS' short poem, the traveller encounters this shape-shifting, "horror-faced" creature. As readers, we are left to imagine how the confrontation will resolve itself, but it seems unlikely that it will end well for the traveller.