The Monsters are really just Father’s first set of children which he adopted and then caged like animals–alongside real animals. His formulas turned them into beasts. Some of these monsters are good such as the Raven and Great Danes. However some are evil and aren’t allowed in the caves. They must hide in the forest.
Father’s first set of experiments didn’t turn out so well. They’re hideous monsters and hide in these caves from everyone. More on these creatures in future books…