Trying to remember a sci-fi fantasy

[Aparecium]

Set in some utopian/dystopian future, where everybody wears a wristwatch-style bracelet that is wired into their body to constantly monitor their health. Its readout says things like ‘Everything is normal’ to reassure the wearer, or directs the wearer to go to the doctor.

Protagonist is a very normal, very ordinary family man who suddenly starts growing a horn on his forehead. Nobody notices anything at first, because their monitors reassure them that ‘everything is normal’, so they can’t have seen what they saw. Eventually the man’s own monitor recognises that he is no longer ‘normal’ and - I think - tells him to report for euthanasia. At which point he escapes and…turns into a unicorn?

I think it might have been a short story.

Does it ring any bells for anyone?

That sounds like the short story "Mythological Beast" by Clifford D. Simak?

Could well be. As I wrote the OP the title ‘Mythical Beast’ came to me, but I couldn’t find anything by Googling. But I still can’t find the story even with your title and author. Only book titles come up, with lists of short stories, and the story I remember does not appear on any of the online synopses I’ve found.

From Gemini:

Based on the specific details you provided—the wrist monitors, the refusal of society to see the horn because the machine says "normal," and the transformation—this story is almost certainly:
"Mythological Beast" by Stephen R. Donaldson
It was originally published in The Magazine of Fantasy & Science Fiction in 1979 and can also be found in his short story collection Daughter of Regals and Other Tales.
Why this fits your description:

• The Protagonist: The main character is named Norman Diel (a play on "Normal"), a very average, law-abiding citizen.
• The Technology: Everyone wears a bio-monitor on their wrist. As long as the monitor reads "Vital Signs Normal," society (and the wearer) is conditioned to believe that nothing is wrong.
• The Horn: Norman wakes up with a lump on his forehead that grows into a distinct horn.
• The "Blindness": This is the central conflict. His wife looks right at him but ignores the horn because checking his wrist readout comforts her that he is fine. He goes to work, and his coworkers do the same. The authority of the machine overrides the evidence of their own eyes.
• The Shift: Eventually, the monitor cannot reconcile the biology anymore. It begins flashing a warning (indicating a genetic mutation or "deviant" status).
• The Climax: Once the machine marks him as "Abnormal," the "medics" (who are essentially a euthanasia squad) come to collect him to "cure" (kill) him.
• The Ending: He flees the city into a wild preserve/forest area. As he runs, he completes his transformation, realizing he now has hooves and fur. He encounters a female unicorn, realizing that is what he has become. > Context: The story is a satirical and somewhat tragic look at how humanity relies on technology to define reality, losing touch with nature and their own senses in the process. > Where to find it If you want to re-read it, looking for "Daughter of Regals" (the collection) is usually the easiest way to track it down, as single issues of 1979 magazines are rare. Would you like me to help you find a digital copy or a similar story about altered percepti ons of reality?

Ah, brilliant, thank you - both! I think I even know where Daughter of Regals is in my parents’ house (if they haven’t cleared it out). My dad used to love sci-fi.

A prescient book, given people’s 21st century fixations with their fitbits and smart rings.

Wild. Thanks for posing the question and thank you to those who had the answer because now I think I'd like to go find the short story

The description rang a bell but I just couldn't place it and to be honest I thought of Anne McCaffery in one of her books of short stories

I now need to find my copy of Daughter of Regals now

Could also do with finding my copy of Donaldsons The Mirror of her Dreams first part of Mordants Need, I have the second book.

Daughter of Regals is £2.99 on Kindle currently (you don't need a Kindle, just use the free app on your phone)