The Monster in the Letter Box: And Other Scary Tales (Michael Dahl's Really Scary Stories)

Dory is brand-new to her neighbourhood, so it's a good thing she has a pet monster to keep her company. When she tells the mailman about her monster, he doesn't believe her. When Dory begins to get angry that he's not taking her seriously, she opens up the mailbox to show him the monster - only to find that the mailbox is empty!