comment: >
  
  I agree with you. I don't think it's supposed to be the perfect or the correct solution, there are many ways, we, animals, develop whatever we need. But you should realise that zebras in Africa share the same predator, lions. They are striped because it helps them to survive as a species. Or tigers are striped because they too hide among the grass to find food. It's beneficial for them. It doesn't matter for European domesticated equines. 
  Exactly, it's about the environment, if it would be necessary, many breeds of horses would have at least signs of keratine, or bone where the horn would be. Because they would have to adapt to their environment, if they don't need it for survival they won't have it. Kind of like humans having a tail bone, we don't need a tail to keep a balance, so we don't have it, yet these little bones are in there. Like an anthelope needing horns to protect itself from other animals, a moose needing antlers to protect itself, an owl needing good eyesight and hearing to find food to survive. Nothing is perfect, but if it's working for these species it stays. 
  We don't have unicorns because horses don't need horns to survive. If there would be a predator that a horn would help with, like something that would attack from above, then maybe. Even more likely. But they don't need it, they have a strong kick to protect themselves. I've never seen a horse kicking upwards, but I've also never seen an anthelope or other horn/antler wielding animal being chased by a flying predator. I may have not seen a lot, though. But anthelopes are not equines, so here's the difference. Domesticated european goats could be afraid of flying predators, although there are not that many. But they still have horns to fight between their own species.
  If it keeps them alive, or at least helps them to survive, they would have it. If they don't need it "why bother".
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  The myth is unicorns have magical abilities not just the horn. >
  
  I was gonna say, Can a giraffe shoot rainbows out of its butt? >
  
  ? Can a unicorn? Didn't know they had this ability. James Bond should ride one >
  
  Mythical means fictitious, not peculiar.  Giraffes are real, and unicorns are not. >
  
  The implication is that myths are compelling, and why would a horse with a horn spellbind so many storytellers when things like giraffes exist. >
  
  b/c giraffes are real >
  
  Imagine the reaction of Europeans when first told about the Platypus. 
  So it's a mammal that suckles it's young which hatch out of eggs and has a bill like a duck. Oh, and it's venomous and lives in the water. >
  
  And it glows in UV light >
  
  Sure but do giraffes have silver blood that grants the drinker immortality? >
  
  Yes >
  
  Can confirm >
  
  "The strange creature has the head and neck of a snake, the body of a leopard, the haunches of a lion, and the feet of a hart." - description of The Questing Beast in Arthurian legend. 
  Almost certainly describes a giraffe or something giraffe-like. >
  
  You would think that in millions of years that there have been horses, just one would have a genetic mutation for a single horn. >
  
  Not really. Even if growing a horn required only a handful of genetic changes, so long as half those changes result in a disfigured mess that won't reproduce, evolution won't produce a single horn. (Flip 6 base pairs, you get a unicorn, but any 3 and you get a mess) Evolution only works in small changes. >
  
  So, a Pegasus would be totally out of the question. >
  
  It depends. Could you selectively breed one, given a million years, probably. (invent genetic engineering, it's faster) Has a flight capable Pegasus ever existed, no. Could a Pegasus have evolved under different circumstances. Maybe.
  Probably not very likely without a somewhat contrived scenario. If horses again and again got thrown off cliffs, with a mixture of sharp rocks and soft pillows at the bottom, such that the only difference between a safe landing and death was a small amount of ability to steer on the way down, then such wings might evolve. (Or you might get the legs producing large skin flaps until they had 4 bat wing style legs.) So even in this scenario, you probably need something else to select for shoulder nubs of some form first. There is sexual selection, small population mutation etc. But sexual selection can make any change that prospective mates can see, and small population mutations can do anything not too lethal. So those don't narrow the genetic changes down much. 
  Or maybe a land with lots of crevasses, all at exactly the width between shoulders, so that slightly wider shoulderblades are the difference between falling down a crevasse and dying and escaping. And then the crevasses slowly stretch at the same rate the shoulders evolve protrusions. >
  
  I think that the bone structure of horses is too thick and heavy. They would need a hundred foot or more wingspan to get off of the ground. >
  
  Evolution will alter the bone structure if being light is a big advantage. >
  
  True, if they became thin and light like birds, than they could fly. >
  
  I read somewhere that Unicorns are the un-official National Animal of Scotland. 
  The reasoning behind why Scotland chose them to be as such, is because in Mythology the Unicorn is the arch nemesis of the Lion. It doesn't make a lot of sense until you find out that the Lion is the official National Animal of Britain...
  I have no idea if that's true, but I do know that a Unicorn is damn near impossible to find for a couple.
  I probably shouldn't put my neck out there though .. >
  
  It's not unofficial, it's official. And the Lion isn't the national animal of Britian but rather the national animal of England. As an aside, the national animal of the Welsh is the dragon. >
  
  Thank you! I couldn't remember it all correctly for sure. I just remembered reading about that and thinking, damn. That says something significant and also sort of humourous about sending a message to England. >
  
  The giraffe was known as the questing beast in Arthurian legends. "The strange creature has the head and neck of a serpent, the body of a leopard, the haunches of a lion and the feet of a hart." >
  
  Unicorns aren't meant to be just horses with horns though that's just how they've become known in the last century or so. A unicorns hooves were supposed to be cloven like a goats instead of a single hoof on each foot. The tail of the unicorn was also meant to be more like that of a cow where you see the appendage with hair at the end instead of the look of a horses tail. Small note they also had fur on the their chin like a beard but that's barely worth noting. Honestly the differences are pretty small but it's still more than horse with horn >
  
  and at the very least they also have magic, also maybe it's just me reading too much fiction that pointlessly-genders stuff like this but I thought only male unicorns had the chin fuzz >
  
  Yeah, you're right! Unicorns may not be as mythical as we thought they were, but they still make for a pretty cool addition to any fantasy world. On the other hand, Giraffes are pretty one-of-a-kind and definitely deserve more recognition for their uniqueness! >
  
  I kid you not, several mythical beasts are speculated to originate from people seeing giraffes and assuming that shit had to be a mythical beast >
  
  You're missing the point entirely. It's not the horn that's unique. It's the magic that unicorns have that makes them special. >
  
  Doesn’t it just make more sense to have a horse with a horn than a horse with neck that’s half it’s height, or a fucking armadillo? >
  
  wait... you really think thoses horses with necks as tall as trees really exist ? >
  
  Bro go to zoo >
  
  I mean come on. There are rhinos and sword fish and narwhals. #PLAUSIBLE >
  
  Every time we discover something that seems like a beast, it’s named as an animal. >
  
  I heard this exact same point on the Game Of Thrones book podcast >
  
  I still can't comprehend some animals..we only see at documentaries. Giraffe, rhino etc. If they didn't exist , they would be called fantasy creatures >
  
  Giraffe has a long neck because they eat trees, lol.
  Unicorn having a horn on it's head would imply that they have to protect themselves from something that could attack and eat them. What would that be? Griffins, perhaps? Or dragons? >
  
  Bears? Wolves? Errant knights trying to prove themselves and win a lady's favour. >
  
  If bears or wolves would be this big of an issue for unicorns, every horse would be a unicorn. The wild ones, at least. I think. I might be wrong, it's evolution and stuff. >
  
  
  it's evolution and stuff.
  
  This isn't exactly the strongest of assertions. Evolution is not a process of "perfection" and there's no "correct solution" that improves a species. Consider the similarities of horses and zebras, these animals have a shared distant ancestor species - but zebras developed camouflage to hide in tall grass. This adaptation is beneficial to them, it reduces their chances of being spotted by African predators. So then why are there still horses? If zebra stripes benefit against predators, why don't all horse-shaped animals have them?
  The answer lies in the most influential part of evolution; the environment. Not all horses are preyed upon by lions in the African savannah, and not all horses are preyed upon by bears and wolves as they would've been in ancient Britain (where unicorn myths originate, I think) so they would develop differently. The interesting thing about evolution is when you put the same species into two different environments, they will adapt (or die) into a species suited for that environment. AND even if a species is in the same type of environment but are separated by a significant distance, then they can develop different adaptations for the same thing, because these groups are distinct and evolution has no correct solutions.
  Thus, in conclusion to my unnecessary long answer, not all horses would become unicorns because not all horses have developed in an environment that could encourage unicorn traits - and those that have, simply didn't, they developed other solutions; woe to us who got faster horses instead of cool unicorns.