the terrible thing

I'm not sure what exactly the Terrible Thing is, but it is surely unwelcoming. I saw the short passage in the beginning of the book, and it seemed to say something about Hitler and The Nazis(if it didn't i apologize for my mistake.) According to that passage, I think the whole story has something to do with the power that has been held in some certain people and the sufferers during world war II(which are kinda like those animals in the story).
I think those rabbits should be much more well-alert after all those other creatures had been caught away by the terrible thing. It's obvious that they are under danger, but regrettably they didn't take the little rabbit's word seriously and instead they just refused to face it and take action.

Well I think humans do the same thing all the time in real life, there are times when we are confronted by those terrible things. What ever they are, we should be brave enough to face 'em. And the most important part, LISTEN CAREFULLY TO THOSE WHOEVER TRYING TO WARN YOU, their opinion might get you through the tough situation.