They're the center of the game in Monsterpocalypse: the giant monsters that you build your army around. This month, two new ones are roaring their way into town and you can get a preview of them here.
From the preview:
Another month means another wave of new Monsterpocalypse releases! Two new monsters and two new elite blisters are coming at the end of this month. So, today is monster day—and Slingoozer and Gakura are both monsters with a fixation on the debris littering the battlefield.