Creatures
Unicorn
A pure silver horse of the forest, its blood can keep a dying man alive but curses him to a half-life. The dead unicorn Harry finds in the Forbidden Forest is being drained by Quirrell under Voldemort.
In the books
Harry Potter and the Philosopher's StoneChapter: 5
Diagon Alley shops sell unicorn horns, and Ollivander describes using unicorn hair in wand cores.