I can see a possible simalrity..........
...Between the Ents in Lord of the Rings and the Centaurs in Harry Potter. What i mean is, the Ents in Lord of the Rings were neutral at the start of the battle for Middle Earth and the reason Saruman was defeated is because he didnt consider the Ents in his plans and didnt think they would oppose him in the war.
I think something similar will happen with the Centaurs in HP, The war will be going badly for the good wizards until the centaurs pick their side, Voldermort will not have featured them in his plans and will be taken be surprise when the fight against him.
I think it will be them that sway the war in book 7.