Oh, you didn't get that? It took me a couple watches. He's an endoskeleton AND a liquid one layered on top that splits like Russian nesting dolls. They cut it in an odd way that makes it seem like one guy, but look at it again, the liquid and the skeleton become separate guys after liquid reforms on the hood of the truck, and jumps at Grace with the rebar. Then Sara shoots liquid with the shotgun, and skeleton with the bazooka.