The Whomping Willow
"I told you, months ago, that the Whomping Willow
was planted the year I came to Hogwarts. The truth is that it was planted
because I came to Hogwarts."
-- Remus J. Lupin
The Whomping Willow is a very valuable, very violent tree planted alone in the middle of the school grounds. It was planted the same year that Lupin arrived at Hogwarts (c. 1971 [Y-9]) to disguise the opening to a secret passage from Hogwarts to the Shrieking Shack. Lupin would go through a hole in the Willow's roots every month to transform into a werewolf in the Shack where he couldn't hurt anyone. The Willow was so dangerous that it kept other people from entering the passage and encountering the werewolf.
No one was ever told what the Willow was really for. What people did know was that it would attack anyone who came within reach of its branches. Harry's Nimbus 2000 broomstick sailed into the Willow and was smashed to bits (PA10). Harry and Ron accidentally crashed the flying Ford Anglia into it, and the tree severely dented and smashed the car before it could reverse out of range of the swinging branches (CS5).
Lupin told Harry that "(p)eople used to play a game, trying to get near enough to touch the trunk. In the end, a boy called Davey Gudgeon nearly lost an eye, and we were forbidden to go near it. No broomstick would have a chance." (PA10). However, there is a way to temporarily paralyze the tree. There is a particular knot on the trunk which can be pressed using a long branch in order to stop the attacks (PA).