Hox genes determine head to tail positioning of appendages, they are not responsible for front to back positioning in bilateral animals (us).

In short: taking an insect as an example: they determine which segments get which appendages (antennae legs pincers) but they don‘t determine where in that segment the appendage appear Rotationally.

Additionally why not just use Rhino horn genes on horses? Doesn‘t say unicorns have tooth based horns.

Also: the narwhal ‚horns‘ (horns actually) are just their canines turned to the front. Usually just one grows outside their head, but two happens as well.

To transpose just canines… from within the box that make up the whole mandibular structure… seems like it would be harder than just creating new life at this point.

Oh btw the Saola, is a deer with a single horn, that is most similar to how unicorns are portrayed in contemporary art.

Just grab one of those for your research.