In several places we hear talk of people questing for unicorns and needing to use a virgin as bait. Legend says that a unicorn is magnetically attracted to a virgin, and will come up and lay its head in her lap. Unicorns, then, represent both adventure (quest time!) and innocence (those virgins, right?). They also represent doin' it—that unicorn's horn ends up in the virgin's lap—but doin' it in basically the most chivalrous, romantic way possible.
This symbolism gets violently turned inside out (almost literally) in The Once and Future King when the G-boys go on a unicorn hunt. What a huge surprise when they actually find one! And what do they do? They freaking butcher it.
In this incident, the unicorn symbolizes a loss of innocence, and taking a step out of childhood and toward adulthood… except these kids have such a dysfunctional relationship to their childhood that they come of age in a gruesome, messed-up fashion.
The unicorn also stands in for Morgause. They initially hunt the unicorn in order to please their Mommy Dearest, and they end up taking their aggression toward their mother out on the unicorn. Eventually, this aggression is directed toward its proper source, and the boys (well, Agravaine, at least) kill her.