The Space Monsters and mutants are characters from the World War X series. As their name suggests, the monsters are a species of space monsters found in outer space which typically have no alliances.
In general, the Space monsters are largely considered antagonists and are almost inaudible like most of the Dinosaurs depicted on the wiki. Some of the mutants however have the ability of speech like their main allies, The Invaders.
Though it appears they are under the control of the aliens, it appears that the monsters and mutants attack the Space Fleet because they see them as a threat to space unlike The Invaders who try to eradicate humanity from existence and their allies who assist them.