The setting of the story might also contribute to its interest. If it's set in a strange and magical world where the rules are different from ours, and the monster fits right into that world, it creates a whole new universe for readers to explore. The story might also have a moral or a lesson hidden within, which can make it not only interesting but also meaningful for readers, especially children who can learn important values from it.
The idea of a monster being someone's own makes it interesting. It's not a common concept.