comment: >
  
  Evolution will alter the bone structure if being light is a big advantage.
context: This is a friendly reminder to read our rules.
  Remember, /r/Showerthoughts is for showerthoughts, not "thoughts had in the shower!"
  (For an explanation of what a "showerthought" is, please read this page.)
  Rule-breaking posts may result in bans. >
  
  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.