Unicorns in minecraft? Not exactly! But in the Middle Ages, real horse armor often times had a metal horn, presumably where unicorns have their horn.
I propose creating new horse armor to reflect this! All we would need to craft this item is an end rod and horse armor. Horse armor itself would remain un-craftable. But once you find an end city and get an end rod, you would be able to put an existing piece of horse armor into the crafting table with an end rod, and get roughly what is shown in the image.
One feature of this end rod horse armor could be that since end rods are light sources, when you place the end rod horse armor on your horse, it would act as a light source (the same way as if you had placed the end rod).
I think this would be a fun addition to the game, and give another interesting purpose to end rods!