No one knew Minerva McGonagall's boggart. Her Patronus was, of course, easy enough to guess, and some claimed to have even seen it, but the Boggart was another matter altogether.
This was a conversation the students often had, right after the lesson where they learnt about the strange creatures. There was invariably quite a diversity in the theories about her Boggart. Some came up with animals, such as tigers or lions. Others laughed it off and claimed that she would be more likely to be afraid of something completely unexpected, such as bords or insects. Yet others decided she was afraid of something more abstract, such as the darkness, or perhaps even Dementors, after the war.
They were all wrong of course. Minerva herself was by no means unaware of the numerous theories going around the within the students and sometimes even a few members of the staff. It was amusing, to an extent.
Yet, she never revealed her Boggart to put an end to their guessing. She was afraid, she knew. She was afraid of what they would think. Her boggart wasn't a terrifying creature or a horrendous abstract idea. It was both. It was a young man, who had the brightest of minds and the darkest of hearts. A young man who she could have saved, should have saved, but hadn't. A man she had loved and yet had watched him turn into a monster. A man who embodied all her regrets.
Minerva McGonagall's boggart was the image of one Tom Riddle, and she would never tell anyone as much.
Notes: For Fruits! I wasn't planning to turn it into Tomerva, but well. I hope you like it though!