The Whomping Willow

A violent tree with a degree of sentience and a tendency to destroy things. Ron Weasley and Harry Potter caused some damage of their own when they crashed a flying Ford Anglia into the Willow while trying to reach the school after missing the Hogwarts Express. It later came to light that Hogwarts' Whomping Willow was not merely a quirky feature of the grounds, but that it served a secret purpose. It was planted to guard a passageway to the Shrieking Shack in Hogsmeade to provide Remus Lupin with safe place for his werewolf transformations.