That's right. In Halo 4, the unicorn is unlocked late in the game and a sign of rarity. Even more rare however is the unicorn helmet in Halo Reach. [url=http://www.halopedian.com/images/8/86/AAHelmet.jpg]Pic[/url]
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At first, I thought you were talking about the FOTUS armour, as it resembles a unicorn of a sort.