Leonardo, the Terrible Monster is a children's book written and illustrated by Mo Willems. It was first published in 2005.
Description[]
Leonardo is truly a terrible monster--terrible at being a monster that is. No matter how hard he tries, he can't seem to frighten anyone. Determined to succeed, Leonardo sets himself to training and research. Finally, he finds a nervous little boy, and scares the tuna salad out of him! But scaring people isn't quite as satisfying as he thought it would be. Leonardo realizes that he might be a terrible, awful monster--but he could be a really good friend.