Hermione, Ron and Harry read Moste Potente Potions to look up the recipe for 'The Polyjuice Potion' while in Moaning Myrtle's bathroom. The ingredients include powdered bicorn horn (CS10).
Hermione steals horn of bicorn (and boomslang skin) from Snape's office during a Potions double lesson on a Thursday afternoon, while Harry creates a distraction by throwing Fred's Filibuster firework into Goyle cauldron of the Swelling Solution. (CS11)
bi = two, corn = horn
A bicorn is a mythological creature which fed only on good, kind husbands, and which was therefore fat and well-fed. The bicorn is described as half panther-half cow with two horns.