This snow-white Unicorn was captured at some point in the past by Scrooge McDuck, and placed in a cell in his Other Bin (the underground complex below McDuck Manor where Scrooge stores his most dangerous belongings).
Though looking the part of the dreamy, girly Unicorn (complete with a cell designed with illusory clouds and rainbows), it is actually a dangerous and murderous beast who wants nothing more than to impale anyone who comes near it with its extremely sturdy (and sharp) horn.
During her and Lena De Spell's dive into the Other Bin, Webby Vanderquack briefly freed the unicorn, and managed to ride it for a few moments (fulfilling an old dream), before it was sealed back within its prison.
The Unicorn served as the inspiration for the Talking Swordhorse, a being who existed within a dream realm created by Lena De Spell's haywire magic and was, of course, born of Webby's imagination. Though physically identical to the "real Sword-Horse" earlier encountered by the duckling, this Unicorn was not in the least aggressive, and was sapient.