Why are there no Unicorns?

Why are there no Unicorns? Perhaps horses develop in a way that cannot be easily modified to produce a Unicorn, so such creatures have never arisen. Or maybe Unicorn-like animals have been born in the past but because there is no advantage for a horse to have a horn, such creatures did not thrive and were weeded out by natural selection.

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<p><a href="http://www.physorg.com/news99303729.html">Why are there no Unicorns?</a></p> <p>Why are there no Unicorns? Perhaps horses develop in a way that cannot be easily modified to produce a Unicorn, so such creatures have never arisen. Or maybe Unicorn-like animals have been born in the past but because there is no advantage for a horse to have a horn, such creatures did not thrive and were weeded out by natural selection.</p>