I am designing a horse with a drill bit shank on top of its head that act as a projectile, my question is what mechanism(s) would be possible for a horse to evolve so it can snipe anything without relying on magic? How does it handle the recoil and can it be on semi or fully automatic if so how does it stores it magazine of horns? No cyborg no magic and yes to genetically augmented mutant.
This is probably a stretch, but what about using the same mechanism the Bombardier beetle uses to shoot hot liquid? It mixes hydroquinone and hydrogen peroxide to make a very hot liquid that it sprays at its enemies. You could add some hardened compartment in the unicorns head that protects the rest of it from the heat. The chamber would be sealed by the horn and the valves to the organs holding the Bombardier beetle chemicals. The pressure buildup would propel the horn.
As far as multiple horns, I'm not really sure how that could work.. Maybe the horn (if it were small) would be regrown after a day or two. Or maybe there could be multiple horns and the "unicorn" (now something else) could pick which one it shot from?