I just hope it's something really cool...like giant insects
Arthropods have several evolutionary ticks (no pun intended) that restrict their size, especially on land. Their armor is heavy, and their lungs don't force air in and out like ours do; they act instead more like a filter, collecting oxygen that happens into their system. In the Carboniferous Period, though, oxygen levels were about 35-45% higher than they are today. There were some freaky creatures: Meganeura, a dragon fly roughly the size of a bald eagle (as seen in that awful Godzilla movie); Mesothelae, a spider roughly the size of a human head; Arthropleura, a millipede that could grow to 6m in length!