Whether we're talking about the long-necked monster in the suburbs (3) or the Hakken-Kraks and their incessant howling (28), there are monsters all over this book. While it's easy to ignore them in familiar environments, they become more and more threatening throughout the book, until finally our intrepid hero conquers them all.
On and on you will hike.
And I know you'll hike far
and face up to your problems
whatever they are. (29)
The monsters, then, are all of those things that threaten your child along the way, and represent all that must be overcome. Rocky style.