HedonismBot

Actually, a prosthetic horn could have fit neatly within Tempest's character arc: Her backstory strongly indicates that her obsession with getting a new horn was more about being normal for the sake of being normal than doing anything magic-related with her life. So you could have Twilight suggest a prosthesis to Tempest, only for her to respond "Screw that, the Storm King is offering me the real thing!", then leave the story as it is until the Storm King goes down, then have Tempest warm up to the idea. There, the presence of a practical remedy would support the movie's message of friendship, not undermine it. Also, it would have been darkly hilarious if there had been a clinical trial of a prosthetic horn as Tempest was joining up with the Storm King.

Then again, "To Where and Back Again" had him affected by anti-magic effects same as any unicorn, as did the Season 10 comic which had him visit Abyssinia.