Nice work Agis! You had asked me for their backstory, so I thought I'd provide it. The space demons are bioconstructs of an alien race from another dimension, the Timonids. The Timonids have been fighting a millenia-old war against a greatly superior alien race, and in their desperation to keep up this struggle have designed the space demons as perfect close combat shock troopers.
Occasionally in the course of these wars Timonid invasion forces travel to our dimension then back to their own in an attempt to "outflank" their enemy, and now and then some of the Timonid bio-transports are stranded in our dimension during these manoeuvres. The transport will then dump its space demon passengers on a planet -- and this is how they have spread in our dimension.
The basic trooper is the assault demon, very fierce in attack but relying on massed assault and higher leadership for prime combat employment. These have ribbed heads. Hammerhead demons support these, spitting acid short distances from two spouts on their heads. There are also inflitrator demons, who have smooth domed heads and operate singly, capable of functioning on their own initiative, often outflanking the enemy or sneaking into vulnerable rear-echelon areas -- or onto commercial freighters. :-)
Leading the assault demon swarms are larger creatures called demon kings -- much larger and highly evolved infiltrator demons which command their troops telepathically. Higher up the command chain are the demon queens who plot higher level tactics but are as powerful as a king in close combat.
Even larger demons exist -- the Trolls, huge mindless beasts which use their enormous strength to batter down defences and tear tanks to pieces, and the Colossi, which stride on three long legs and can spit gobs of acid long distances.
Largest of all space demons is the dragon, also called a great queen, which is spawned when a world is almost conquered. This creature will help to annihiliate the last, most tenacious resistance, then flies into space, seeking an Einstein-Rosen Bridge so that it can spread the species to new star systems.