Why is the Scotland official animal an Unicorn?

In Celtic Mythology the Unicorn of Scotland symbolised innocence and purity, healing powers, joy and life itself. It was seen as a symbol of masculinity and power.

It isn't known when exactly the Unicorn of Scotland first appeared as a Scottish heraldic symbol , however, the two Unicorns that appear as part of the 'Royal Coat of Arms' at Rothesay Castle are believed to have been carved sometime before the 15th Century.