Mad Eye's Boggart
Moody still thought of what he was told in defence of the dark arts class.
"No one knows what a boggart looks like in his natural state as he always changes to what he thinks will scare us the most."
The first thing he thought when his eye got replaced with the magical one after a particularly vicious fight with a death eater was wondering what a boggart would look like if he didn't know he was being watched.
He spent weeks combing the wizarding world looking in all the dark spaces he could just hoping to find one. He always looked through the wood of cabinets and inside clocks and trunks but was always unsuccessful.
Two months after he stopped looking he checked the grandfather clock which stood in his hall just to see if it needed winding. To his surprise there was something else in there, he couldn't quite make out the shape. When he could focus enough to make out its shape it was like a hairy goblin but at half the size, and with its head proportional to its body size, but covered in tiny sprouts of hair at its elbows and knees and out of the top of its head.
Moody opened the clocks case and out came his original boggart of himself dead, his greatest fear is that one time he would not be vigilant enough. However instead of banishing it instantly, Moody looked at it and said "I have seen your natural state, it's kind of pathetic, is that why you feel the need to change shape?"
The next thing Moody knew, the boggart had transformed not into the creature he saw in the clock but into a creature which the closest thing Moody could think of to describe it would be a muggle toy he saw in a shop window once called a furby.
When he saw this, Moody laughed and the boggart disappeared into a wisp of smoke without the need of the ridickulus spell.
After this, he never told anyone what a boggart looks like as he always felt that an alert wizard was a safe wizard and taking one more threat out of the world would do more harm then good.