Not sure why Poland would be considered a 'darkhorse' they are 10th in the FIFA rankings, FWIW, have a pretty solid squad which includes one of the best players in the world in Lewandowski.
I'm sure most people would view them and Columbia as the class of Group H.
A darkhorse to me, would be a country that could pull of what Costa Rica did 4 years ago in Brazil when they topped a difficult group that contained Italy, Uruguay and England and went on to the QF losing in penalties to Holland.
I think most 'experts' had them tipped to be rooted to the bottom of that group.
Who is a nation most would figure to be gone in group play that could go on a run.
As for Croatia, I could certainly see them finishing 3rd in their group behind Argentina and Nigeria. Just depends on the mood their in. I think the players like Dalic but we'll see what kind of tactician he is in a few weeks time. I just wonder about their backline, it's troublesome to me.