g to our man in Rome:
...but they have planned a snare for the fish, and get the better of them by their fisherman's craft.
They fasten red (crimson red) wool around a hook, and fix onto the wool two feathers which grow under a cock's wattles, and which in colour are like wax.
So you could argue on two counts that this fly is not a Hippouros imitation, in which case, it is only of theoretical interest what the Hippouros was.
If we do think it is a Hippouros imitation, then we have to square the difficult circle of reconciling a pattern with a red body with a natural fly which had a yellow body. The only way I can think of doing this is if there is a fly which changes its body colour from yellow to red, but if we think the Hippouros is being imitated here, we also have to consider Aelian's "hovering". The biggest problem here is that he was describing all this third hand. It might be that he was actually describing the ascent of duns - Ephemera danica duns and most spinners look as if they are hovering. On the other hand, it might be that he was describing a truly hovering fly; but I can't think of any patterns anywhere which imitate this sort of insect, chiefly because they are hardly of any importance to fish in their diet.
If you want to know about some interesting attempts to guess what the Macedonian fly looked like, click here.