Muggles' Guide to Harry Potter/Magic/Unicorn

Overview
The unicorn is a majestic horse-like creature with a single horn on its head. When fully grown, they are pure white, but are golden when foals and turn silver before achieving maturity.

Extended Description
We first hear something about Unicorns in Harry Potter and the Philosopher's Stone, when Harry, Hermione, Neville Longbottom, and Draco Malfoy have to serve a detention. Hagrid tells them that something has been wounding and killing unicorns in the Forbidden Forest, and the four of them, with Hagrid and his boarhound Fang, are going to see if they can find out what it is. We learn first that unicorn blood is silvery, and then later that unicorn blood can be used to keep one alive, no matter how close to death they are.

Harry, Ron and Hermione witness a unicorn during a lesson of Care of Magical Creatures taught by a temporary teacher, Professor Grubbly-Plank, in their fourth year. Unicorns apparently prefer a female's touch.

In Harry Potter and the Half-Blood Prince, we learn that Hagrid has been collecting unicorn tail and mane hairs that get caught in the bushes. He says they are dead useful for tying things up, very strong. Eventually, in a burst of drunken generosity, he gives the bundle of them to Horace Slughorn, who apparently thinks they are worth about ten Galleons a hair.

A unicorn's horn, blood and hair have highly magical properties. Some wands are manufactured containing unicorn tail hair.