" (I)t is a monstrous thing, to slay a unicorn. Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips."
     -- Firenze the Centaur

References:

• silver unicorn horns cost twenty-one Galleons each at the Apothecary in Diagon Alley (SS5)
• Mr. Ollivander uses unicorn tail hairs in some of his wands (including Cedric Diggory's and Ron Weasley's). (SS5)
• Unicorn blood is silver in color and shines under moonlight (SS
• Unicorns are powerful magical creatures. It's not easy to catch them (SS, PA
• long slender legs, pearly-white mane.
• horn and tail hair used in Potions
• "That is because it is a monstrous thing, to slay a unicorn. Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips." Firenze the Centaur
• So brightly white it makes snow look gray.
• Unicorn hoofs are golden.
• Unicorns prefer a woman's touch.
• Unicorn foals are pure gold. They turn silver when they're about two years old. They grow horns at around four. At about seven years old, they're fully grown and turn pure white.
• "They're a bit more trustin' when they're babies...don' mind boys so much...." -- Hagrid
• Voldemort's potion: unicorn blood and snake venom
• Ronan states, when the unicorn is killed, that it is always the innocent that are the first to die. Interestingly, the two people who we know have unicorn hairs in their wands are Cedric Diggory and Ron Weasley. Cedric Diggory was killed, an innocent bystander who got in Voldemort's way. Is Ron also fated to die?